[Hexagon] Handle lowering of SETCC via setCondCodeAction

It was expanded directly into instructions earlier. That was to avoid
loads from a constant pool for a vector negation: "xor x, splat(i1 -1)".
Implement ISD opcodes QTRUE and QFALSE to denote logical vectors of
all true and all false values, and handle setcc with negations through
selection patterns.

llvm-svn: 324348