Fix JIT over reporting RCPC2

[dhartglassMSFT]

Lowering::TryCreateAddrMode would call compOpportunisticallyDependsOn(InstructionSet_Rcpc2) for volatile loads/stores on ARM64, but could still bail out if, for example, the address offset doesn't fit in 9 bits.

This could cause R2R to over-report needing RCPC2 ISA for methods which didn't end up using the feature

Fix is to move the compOpportunisticallyDependsOn after the eligibility checks in the lowering method. Also needed an explicit bailout for scalar-stores that ended up turning into write-barriers, so we don't report an RCPC2 requirement for this method:

public static void WriteReference(ReferenceField value, object data) => value.Value = data;

The bug is somewhat moot now that RCPC2 is removed from the optimistic arm64 set, but better to fix the over-reporting anyway. Got 0 diffs.

Tested by adding RCPC2 back to the optimistic set and building for linux arm64. The methods from the bug no longer had RCPC2 erroneously reported.

fixes #133671

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[tannergooding]

LGTM. There's probably a few others like this too, I've seen too many cases of the opportunistic check happening too early relative to its use.