[TargetLower] Update shouldFormOverflowOp check if math is used.

On some targets, like SPARC, forming overflow ops is only profitable if
the math result is used: https://godbolt.org/z/DxSmdB
This patch adds a new MathUsed parameter to allow the targets
to make the decision and defaults to only allowing it
if the math result is used. That is the conservative choice.

This patch also updates AArch64ISelLowering, X86ISelLowering,
ARMISelLowering.h, SystemZISelLowering.h to allow forming overflow
ops if the math result is not used. On those targets using the
overflow intrinsic for the overflow check only generates better code.

Reviewers: nikic, RKSimon, lebedev.ri, spatel

Reviewed By: lebedev.ri

Differential Revision: https://reviews.llvm.org/D74722