[X86] Revisit the fix I made years ago to make 'xchgl %eax, %eax' not encode...

[X86] Revisit the fix I made years ago to make 'xchgl %eax, %eax' not encode using the 0x90 encoding in 64-bit mode.

Prior to this we had a separate instruction and register class that excluded eax to prevent matching the instruction that would encode with 0x90.

This patch changes this to just use an InstAlias to force xchgl %eax, %eax to use XCHG32rr instruction in 64-bit mode. This gets rid of the separate instruction and register class.

llvm-svn: 322532