[x86] avoid infinite loop from SoftenFloatOperand (PR34866)

Legalization of fp128 assumes things that we should have asserts for,
so that's another potential improvement.

Differential Revision: https://reviews.llvm.org/D38771


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@315485 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed