[RISCV] Merge Base Offset for SFB Pseudos (#187620)
This implements the Merge Base Offset pass for the SFB Load Pseudos.
These Pseudos are expanded after Merge Base Offset, so the pass needs to
handle them.
I also had to extend support in MergeBaseOffset to ensuring that ImmOp
could be a Constant Pool Index, which seemed to be supported in some
checks but not others.
