[X86] Remove a couple places where we unnecessarily pass 0 to the EmitPriority of...

[X86] Remove a couple places where we unnecessarily pass 0 to the EmitPriority of some FP instruction aliases. NFC

As far as I can tell we already won't emit these aliases due to an operand count check in the tablegen code. Removing these because I couldn't make sense of the inconsistency between fadd and fmul from reading the code.

I checked the AsmMatcher and AsmWriter files before and after this change and there were no differences.

llvm-svn: 352608