[Matrix] Preserve existing fast-math flags during lowering

This patch makes it so, floating-point instructions created in
LowerMatrixIntrinsics retain fast-math flags from instructions that are
higher up the chain.

Fixes https://bugs.llvm.org/show_bug.cgi?id=49738

Reviewed By: fhahn

Differential Revision: https://reviews.llvm.org/D103233