[TargetLowering] fix isConstTrueVal to account for build vector truncation

Build vectors have magical truncation powers, so we have things like this:

v4i1 = BUILD_VECTOR Constant:i32<1>, Constant:i32<1>, Constant:i32<1>, Constant:i32<1>
v4i16 = BUILD_VECTOR Constant:i32<1>, Constant:i32<1>, Constant:i32<1>, Constant:i32<1>

If we don't truncate the splat node returned by getConstantSplatNode(), then we won't find 
truth when ZeroOrNegativeOneBooleanContent is the rule.

Differential Revision: https://reviews.llvm.org/D32505

llvm-svn: 301408