[InstSimplify] teach SimplifySelectInst() to fold more vector selects

Call ConstantFoldSelectInstruction() to fold cases like below

select <2 x i1><i1 true, i1 false>, <2 x i8> <i8 0, i8 1>, <2 x i8> <i8 2, i8 3>

All operands are constants and the condition has mixed true and false conditions.

Differential Revision: https://reviews.llvm.org/D38369

llvm-svn: 314741