[LegalizeVectorTypes][X86][ARM][AArch64][PowerPC] Don't use SplitVecOp_TruncateHelper...

[LegalizeVectorTypes][X86][ARM][AArch64][PowerPC] Don't use SplitVecOp_TruncateHelper for FP_TO_SINT/UINT.

SplitVecOp_TruncateHelper tries to promote the result type while splitting FP_TO_SINT/UINT. It then concatenates the result and introduces a truncate to the original result type. But it does this without inserting the AssertZExt/AssertSExt that the regular result type promotion would insert. Nor does it turn FP_TO_UINT into FP_TO_SINT the way normal result type promotion for these operations does. This is bad on X86 which doesn't support FP_TO_SINT until AVX512.

This patch disables the use of SplitVecOp_TruncateHelper for these operations and just lets normal promotion handle it. I've tweaked a couple things in X86ISelLowering to avoid a few obvious regressions there. I believe all the changes on X86 are improvements. The other targets look neutral.

Differential Revision: https://reviews.llvm.org/D54906

llvm-svn: 347593