[VE] Support atomic exchange instructions

Support atomic exchange and atomic compare and exchange instructions.
Change CAS and TS1AM instructions for ISel patterns.  Add selectADDRzi
pattern for them.  Add TS1AM pseudo instruction also for better ISel.
Add shouldExpandAtomicRMWInIR() function to expand all atomicrmw
instructions except atomicrmw xchg.  Add custom lower for i8/i16
atomicrmw xchg.  Modify replaceFI to support CAS/TS1AM instructions
which use "reg+disp" operands instead of "reg+imm+disp" operands.
And, add several regression tests to check the correctness.

Reviewed By: simoll

Differential Revision: https://reviews.llvm.org/D93161