Detect a run of memory tagging instructions for adjacent stack frame slots,
and replace them with a shorter instruction sequence
- replace STG + STG with ST2G
- replace STGloop + STGloop with STGloop
This code needs to run when stack slot offsets are already known, but before
FrameIndex operands in STG instructions are eliminated; that's the
reason for the new hook in PrologueEpilogue.
This change modifies STGloop and STZGloop pseudos to take the size as an
immediate integer operand, and base address as a FI operand when
possible. This is needed to simplify recognizing an STGloop instruction
as operating on a stack slot post-regalloc.
This improves memtag code size by ~0.25%, and it looks like an additional ~0.1%
is possible by rearranging the stack frame such that consecutive STG
instructions reference adjacent slots (patch pending).