Treat undef as unknown, and poison as ignorable.

Try to avoid expensive checks failures from reporting no changes
when some dead instructions were introduced.

Make codegen emit correctly rounded sqrt by default.

Emit the fast but only kind of fast expansion in AMDGPUCodeGenPrepare
based on !fpmath, like the fdiv case. Hack around visitation ordering
problems from AMDGPUCodeGenPrepare using forward iteration instead of
a well behaved combiner.

https://reviews.llvm.org/D158129

This was working around the lack of operator| on FastMathFlags. We
have that now which revealed the bug.

We were basing the defer the fast case to codegen based on the fdiv
itself, and not looking for a foldable sqrt input.

https://reviews.llvm.org/D158127

Extends the new frexp scaled reciprocal to the general case. The
reciprocal case is just the same thing when frexp of 1 is constant
folded. Could probably clean up the code to rely on that constant
folding.

Improves results for the IEEE path for the default OpenCL division. We
used to only emit the fdiv.fast intrinsic with a 2.5 ulp accuracy
threshold with DAZ, which uses explicit range checks. This gives us a
better fast option with the default IEEE behavior.

NFC-ish. Does trigger some reordering of the fdiv scalarization. Also
skips scalarizing in more cases where nothing was going to happen. We
can still scalarize in some no-op edge cases.

https://reviews.llvm.org/D155740