[SLP][X86] Improve reordering to consider alternate instruction bundles

During the reordering transformation we should try to avoid reordering bundles
like fadd,fsub because this may block them being matched into a single vector
instruction in x86.
We do this by checking if a TreeEntry is such a pattern and adding it to the
list of TreeEntries with orders that need to be considered.

Differential Revision: https://reviews.llvm.org/D125712