[AArch64] Standardize suffixes for LSE Atomics mnemonics (NFCI)

This NFC changeset standardizes the suffixes used for LSE Atomics
instructions.

It changes the existing suffixes - 'b', 'h', 's', 'd' - to the existing
standard 'B', 'H', 'W' and 'X'.

This changeset is the result of the code review discussion for D35319.

Patch by: steleman

Differential Revision: https://reviews.llvm.org/D35927

llvm-svn: 309384