AMDGPU: Perform libcall recognition to replace fast OpenCL pow (#182135)

If a float-typed call site is marked with afn, replace the 4
flavors of pow with a faster variant.

This transforms pow, powr, pown, and rootn to __pow_fast,
__powr_fast, __pown_fast, and __rootn_fast if available. Also
attempts to handle all of the same basic folds on the new fast
variants that were already performed with the base forms. This
maintains optimizations with OpenCL when the device libs unsafe
math control library is deleted. This maintains the status quo
of how libcalls work, and only handles 4 new entry points. This
only helps with the elimination of the control library, and not
general libcall emission problems.

This makes no practical difference for HIP, which is the status
quo for libcall optimizations. AMDGPULibCalls recognizes the OpenCL
mangled names. e.g., OpenCL float "pow" is really _Z3powff but the
HIP provided function "powf" is really named _ZL4powfff, and std::pow
with float is _ZL3powff. The pass still runs for HIP, so by accident
if you used the OpenCL mangled function names, this would trigger.

Since the functions cannot yet be relied on from the library,
introduce a temporary module flag check. I'm not planning on emitting
it anywhere and it's a poor substitute for versioning the target.