[X86] Remove duplicate FMA patterns from the isel table.

This reorders some patterns to get tablegen to detect them as duplicates. Tablegen only detects duplicates when creating variants for commutable operations. It does not detect duplicates between the patterns as written in the td file. So we need to ensure all the FMA patterns in the td file are unique.

This also uses null_frag to remove some other unneeded patterns.

llvm-svn: 312470