This patch addresses the signed/zero extension of poison by using a
poison value of the extended type instead of a constant zero of the
extended type.

Inspired by #90738 (although that is a clang codegen issue)

Also handles possible truncations from i32 to i16.

Cleans up some of the poor codegen identified in #98630

This pattern tends to appear during f16 -> f32 promotion

Partially addresses the unnecessary XMM->GPR->XMM moves when working with f16 types (#107086)

Previously we created an FP_TO_FP16 and legalized it in
SoftenFloatOp_FP_ROUND. This caused i16 to be sent to call lowering
instead of f16. This results in the ABI not being followed if f16 is
supposed to be passed in a different register than i16.

Looking at the libgcc binary for the library function it appears the value
is returned in xmm0 so the X86 test was being miscompiled before.

Fixes #107607.

This is to fix #115710.

This reverts commit ababfee78714313a0cad87591b819f0944b90d09.

Add X86 FP80 check.

Reverts llvm/llvm-project#109116

This fixes #108936, but the calling convention doesn't match with GCC. I
doubt we have such a lib function for now, so leave the calling
convention as is.

In cases (like the ones added in the tests) where the condition of a
masked load or store is a splat but not a constant (that is, a masked
operation is being used to implement patterns like "load if the current
lane is in-bounds, otherwise return 0"), optimize the 'scalarized' code
to perform an aligned vector load/store if the splat constant is true.

Additionally, take a few steps to preserve aliasing information and
names when nothing is scalarized while I'm here.

As motivation, some LLVM IR users will genatate masked load/store in
cases that map to this kind of predicated operation (where either the
vector is loaded/stored or it isn't) in order to take advantage of
hardware primitives, but on AMDGPU, where we don't have a masked load or
store, this pass would scalarize a load or store that was intended to be
- and can be - vectorized while also introducing expensive branches.

Fixes #104520

Pre-commit tests at #104527