Use StringRef::npos for StringRef instead of std::string one
Don't use getSetCCResultType for creating a vselect

The vselect mask isn't a setcc.

This breaks in the case when the result of getSetCCResultType
is larger than the vector operands

e.g. %tmp = select i1 %cmp <2 x i8> %a, <2 x i8> %b
when getSetCCResultType returns <2 x i32>, the assertion
that the (MaskTy.getSizeInBits() == Op1.getValueType().getSizeInBits())
is hit.

No test since I don't think I can hit this with any of the current
targets. The R600/SI implementation would break, since it returns a
vector of i1 for this, but it doesn't reach ExpandSELECT for other
reasons.