Only allow replacements of nodes that have a single user. This is better as
simple instructions (e.g. XGRK) are one cycle faster, and it helps in cases
where both inputs share a common node.
Review: Ulrich Weigand
Add support for _FLT_ROUNDS_ in SystemZ.
Patch by Tulio Magno Quites Machado Filho.
Reviewed By: Ulrich Weigand
Differential Revision: https://reviews.llvm.org/D140988