[X86] Remove truncation handling from EmitTest. Replace it with a DAG combine.

I'd like to try to move a lot of the flag matching out of EmitTest and push it to isel or isel preprocessing. This is a step towards that.

The test-shrink-bug.ll changie is an improvement because we are no longer interfering with test shrink handling in isel.

The pr34137.ll change is a regression, but the IR came from -O0 and was not reduced by InstCombine. So it contains a lot of redundancies like duplicate loads that made it combine poorly.

llvm-svn: 349315