[InstCombine] Avoid incorrect folding of select into phi nodes when incoming element...

[InstCombine] Avoid incorrect folding of select into phi nodes when incoming element is a vector type

Summary:
We are incorrectly folding selects into phi nodes when the incoming value of a phi
node is a constant vector. This optimization is done in `FoldOpIntoPhi` when the
select condition is a phi node with constant incoming values.
Without the fix, we are miscompiling (i.e. incorrectly folding the
select into the phi node) when the vector contains non-zero
elements.
This patch fixes the miscompile and we will correctly fold based on the
select vector operand (see added test cases).

Reviewers: majnemer, sanjoy, spatel

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D31189

llvm-svn: 298845