timo micro-optimizes a surprisingly expensive function

perf report before and after

before:

// random context
   7.82%  moar             libmoar.so            [.] MVM_interp_run
   4.44%  moar             libmoar.so            [.] MVM_frame_dispatch
   3.75%  moar             libmoar.so            [.] MVM_disp_program_run
   3.34%  moar             libmoar.so            [.] nqp_nfa_run
   2.60%  moar             libmoar.so            [.] MVM_fixed_size_alloc
   2.51%  moar             libmoar.so            [.] MVM_spesh_arg_guard_run
   2.17%  moar             libmoar.so            [.] allocate_frame
//[...]
   1.35%  moar             [JIT] tid 2084179     [.] MATCH(gen/moar/stage2/QRegex.nqp:2152)
//[...]
   0.40%  moar             [JIT] tid 2084179     [.] _ws(/home/timo/perl6/rakudo/blib/Perl6/Grammar.moarvm:1)
//[...]
   0.34%  moar             [JIT] tid 2084179     [.] termish(gen/moar/Grammar.nqp:3711)

   0.32%  moar             [JIT] tid 2084179     [.] !alt(gen/moar/stage2/QRegex.nqp:1743)
// important bit
   0.30%  moar             [JIT] tid 2084179  [.] linefileof(gen/moar/stage2/NQPHLL.nqp:2499)
   0.05%  moar             [JIT] tid 2084179  [.] line_and_column_of(gen/moar/stage2/NQPHLL.nqp:2439)
   0.01%  moar             [JIT] tid 2084179  [.] linefileof(gen/moar/stage2/NQPHLL.nqp:2499)
   0.00%  moar             [JIT] tid 2084179  [.] lineof(gen/moar/stage2/NQPHLL.nqp:2495)


after:

// random context
   7.85%  moar             libmoar.so            [.] MVM_interp_run
   4.46%  moar             libmoar.so            [.] MVM_frame_dispatch
   3.50%  moar             libmoar.so            [.] MVM_disp_program_run
   3.42%  moar             libmoar.so            [.] nqp_nfa_run
   2.65%  moar             libmoar.so            [.] MVM_fixed_size_alloc
   2.47%  moar             libmoar.so            [.] MVM_spesh_arg_guard_run
   2.14%  moar             libmoar.so            [.] allocate_frame
//[...]
   1.42%  moar             [JIT] tid 2082768     [.] MATCH(gen/moar/stage2/QRegex.nqp:2152)
//[...]
   0.44%  moar             [JIT] tid 2082768     [.] _ws(/home/timo/perl6/rakudo/blib/Perl6/Grammar.moarvm:1)
//[...]
   0.37%  moar             [JIT] tid 2082768     [.] termish(gen/moar/Grammar.nqp:3711)
//[...]
   0.33%  moar             [JIT] tid 2082768     [.] !alt(gen/moar/stage2/QRegex.nqp:1743)
//[...]
   0.29%  moar             [JIT] tid 2082768     [.] visit_children(gen/moar/Optimizer.nqp:2832)
//[...]
   0.27%  moar             [JIT] tid 2082768     [.] prepare-hash(gen/moar/stage2/NQPCORE.setting:402)
//[...]
   0.26%  moar             [JIT] tid 2082768     [.] !cursor_start(gen/moar/stage2/QRegex.nqp:1481)
//[...]
   0.25%  moar             [JIT] tid 2082768     [.] compile_var(gen/moar/stage2/QAST.nqp:6534)
//[... many many many lines ...]
   0.06%  moar             [JIT] tid 2082768     [.] set(gen/moar/stage2/QASTNode.nqp:142)
   0.06%  moar             [JIT] tid 2082768     [.] lexical_vars_to_locals(gen/moar/Optimizer.nqp:711)
   0.06%  moar             [JIT] tid 2082768     [.] statementlist(gen/moar/Actions.nqp:1489)
   0.05%  moar             [JIT] tid 2082768     [.] MARKER(gen/moar/stage2/NQPHLL.nqp:1437)
   0.05%  moar             [JIT] tid 2082768     [.] release_register(gen/moar/stage2/QAST.nqp:5046)
   0.05%  moar             [JIT] tid 2082768     [.] !cursor_start(gen/moar/stage2/QRegex.nqp:1481)
//[...]
   0.05%  moar             [JIT] tid 2082768  [.] line_and_column_of(gen/moar/stage2/NQPHLL.nqp:2439)
   0.01%  moar             [JIT] tid 2082768  [.] linefileof(gen/moar/stage2/NQPHLL.nqp:2499)
   0.01%  moar             [JIT] tid 2082768  [.] linefileof(gen/moar/stage2/NQPHLL.nqp:2499)
   0.00%  moar             [JIT] tid 2082768  [.] lineof(gen/moar/stage2/NQPHLL.nqp:2495)