Merging r315485:

Merging r315485:

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r315485 | spatel | 2017-10-11 11:24:21 -0700 (Wed, 11 Oct 2017) | 7 lines

[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

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/branches/release_50@316607 91177308-0d34-0410-b5e6-96231b3b80d8