[Constant] Add 'isElementWiseEqual()' method

[Constant] Add 'isElementWiseEqual()' method

Promoting it from InstCombine's tryToReuseConstantFromSelectInComparison().

Return true if this constant and a constant 'Y' are element-wise equal.
This is identical to just comparing the pointers, with the exception that
for vectors, if only one of the constants has an `undef` element in some
lane, the constants still match.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@369842 91177308-0d34-0410-b5e6-96231b3b80d8