[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.

llvm-svn: 369842