Many tests use store to undef as a placeholder use, so just replace
all of these with poison.

This is the bulk update with perl, with cases which require additional
update left for later.

DAGISel uses CopyToReg/CopyFromReg to lower PHI nodes. With large PHIs, this can result in poor codegen.
This is because it introduces a need to have a build_vector before copying the PHI value, and that build_vector may have many undef elements. This can cause very high register pressure and abnormal stack usage in some cases.

This scalarization/phi "break-up" can be easily tuned/disabled through CL options in case it's not beneficial for some users.
It's also only enabled for DAGIsel and GlobalISel handles PHIs much better (as it works on the whole function).

This can both scalarize (break a vector into its elements) and simplify (break a vector into smaller, more manageable subvectors) PHIs.

Fixes SWDEV-321581

Reviewed By: kzhuravl

Differential Revision: https://reviews.llvm.org/D143731

This avoids recomputing it between regalloc runs when allocation is
split, and also avoids a debug info test regression.

Switch to using the denormal-fp-math/denormal-fp-math-f32 attributes.

llvm-svn: 341165