[DAGCombine] Allow FMA combine with both FMA and FMAD

Without this change only the preferred fusion opcode is tested
when attempting to combine FMA operations.
If both FMA and FMAD are available then FMA ops formed prior to
legalization will not be merged post legalization as FMAD becomes
the preferred fusion opcode.

Reviewed By: foad

Differential Revision: https://reviews.llvm.org/D108619