This patch changes how LLVM handles the accumulator/start value
in the reduction, by never ignoring it regardless of the presence of
fast-math flags on callsites. This change introduces the following
new intrinsics to replace the existing ones:
llvm.experimental.vector.reduce.fadd -> llvm.experimental.vector.reduce.v2.fadd llvm.experimental.vector.reduce.fmul -> llvm.experimental.vector.reduce.v2.fmul
and adds functionality to auto-upgrade existing LLVM IR and bitcode.