JIT: Fix debug igDataSize assignment to use code size (so exclude gc vars)

[BoyBaykiller]

This random method (and other) tells us at that igData points to after the gc vars.

runtime/src/coreclr/jit/emit.h

Lines 352 to 360 in f2d5042

	VARSET_VALRET_TP igGCvars() const
	{
	assert(igFlags & IGF_GC_VARS);
	BYTE* ptr = (BYTE*)igData;
	ptr -= sizeof(VARSET_TP);
	return *(VARSET_TP*)ptr;
	}

More specifically it points to "addr of instruction descriptors". However the corresponding debug-only igDataSize was being assigned gs which is the size of the entire IG (including gc vars). It should use code size (sz) instead.

Fixes this assert I was hitting in #128536:

runtime/src/coreclr/jit/emit.cpp

Line 10081 in f2d5042

assert(emitLastInsIG->igData + emitLastInsIG->igDataSize == (BYTE*)emitLastIns + emitSizeOfInsDsc(emitLastIns));

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[BoyBaykiller]

@jakobbotsch PTAL.