Hexagon - Add peephole optimizations for zero extends.

	* lib/Target/Hexagon/HexagonInstrInfo.td: Add patterns to combine a
	sequence of a pair of i32->i64 extensions followed by a "bitwise or"
	into COMBINE_rr.
	* lib/Target/Hexagon/HexagonPeephole.cpp: Copy propagate Rx in the
	instruction Rp = COMBINE_Ir_V4(0, Rx) to the uses of Rp:subreg_loreg.
	* test/CodeGen/Hexagon/union-1.ll: New test.
	* test/CodeGen/Hexagon/combine_ir.ll: Fix test.