Convert numeric string array keys to integers correctly in JITted code

Convert numeric string array keys to integers correctly in JITted code

While fixing bugs in mbstring, one of my new test cases failed with a strange
error message stating: 'Warning: Undefined array key 1...', when clearly the
array key had been set properly.

GDB'd that sucker and found that JIT'd PHP code was calling directly into
`zend_hash_add_new` (which was not converting the numeric string key to an
integer properly). But where was that code coming from? I examined the disasm,
looked up symbols to figure out where call instructions were going, then grepped
the codebase for those function names. It soon became clear that the disasm I
was looking at was compiled from `zend_jit_fetch_dim_w_helper`.