Merging r230564:

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r230564 | spatel | 2015-02-25 17:46:08 -0500 (Wed, 25 Feb 2015) | 8 lines

only propagate equality comparisons of FP values that we are certain are non-zero

This is a follow-on to r227491 which tightens the check for propagating FP
values. If a non-constant value happens to be a zero, we would hit the same
bug as before.

Bug noted and patch suggested by Eli Friedman.

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llvm-svn: 236499