[FPEnv][LegalizeTypes][LegalizeDAG][AArch64] Few fixes/improvements for...

[FPEnv][LegalizeTypes][LegalizeDAG][AArch64] Few fixes/improvements for legalizing fp<->int conversion nodes.

This started with adding a test to support get code coverage on
ScalarizeVecOp_UnaryOp_StrictFP by copying an existing AArch64 test
and using constrained sitofp/uitofp intrinsics.

This found 3 separate issues:
-ScalarizeVecOp_UnaryOp_StrictFP needs to do its own replacement
 because the caller can't handle replacing multiple results.
-Missing integer promotion support for sitofp/uitofp
-Chain result not always assigned in ExpandLegalINT_TO_FP.

Committing them together so I can add the test case.