Merging r373397:

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r373397 | ctopper | 2019-10-01 14:55:55 -0700 (Tue, 01 Oct 2019) | 8 lines

[X86] convertToThreeAddress, make sure second operand of SUB32ri is really an immediate before calling getImm().

It might be a symbol instead. We can't fold those since we can't
negate them.

Similar for other SUB with immediates.

Fixes PR43529.
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