[PATCH] Fix instcombine's folding of select into phi nodes for ConstantExpr

[Duncan P. N. Exon Smith]

This patch fixes PR18143.

InstCombiner::FoldOpIntoPhi assumed that Constant::isNullValue can check for false, but this isn’t true for ConstantExpr.  It appears that this bug was latent until some time after 3.2, when the ConstantExpr in question stopped being constant-folded.

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