[X86] Remove unnecessary 'In64BitMode' predicate for instructions that already indicate use of REX.W.
[X86] Don't use PS prefix on LDMXCSR/STMXCSR.

Near as I can tell prefixes are ignored on these instructions except for a comment in the Intel docs about 0xf3. Binutils disassembler seems to ignore prefixes on these instructions. Our disassembler still doesn't distinguish PS and "no prefix" well enough for this to make a functional change, but it helps with experiments I'm doing on a potential new disassembler table builder.
[X86] Use correct opsize on indirect call and jump aliases.
[PowerPC] Use MCPhysReg for tables of registers. Const-correct the tables. Only put the anonymous namespace around classes. NFC.