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: Unde

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`.

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