[DirectX] Expand {u,s}mul.with.overflow in DXILIntrinsicExpansion (#207297)
DXIL has no op for the llvm.{u,s}mul.with.overflow intrinsics. These can
be emulated by performing the full multiply using double-width values
and then checking the high-part of the result. However, this should be
avoided for 32-bit values since we don't want to make the shader start
using 64-bit values if it wasn't before. In this case we can use the
UMul and IMul DXIL operations that return the result as separate low &
high values.
Wider (64-bit) multiplies, which can't widen further, compute the high
half with same-width arithmetic instead.
Fixes #207090
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Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>
Co-authored-by: Farzon Lotfi <farzonl@gmail.com>
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