[NVPTX] Remove `NVPTX::IMAD` opcode, and rely on intruction selection only (#121724)

I noticed that NVPTX will sometimes emit `mad.lo` to multiply by 1, e.g.
in https://gcc.godbolt.org/z/4j47Y9W4c.

This happens when DAGCombiner operates on the add before the mul, so the
imad contraction happens regardless of whether the mul could have been
simplified.

To fix this, I remove `NVPTXISD::IMAD` and only combine to mad during
selection. This allows the default DAGCombiner patterns to simplify
the graph without any NVPTX-specific intervention.