amdgcn: Swap mov<mode>_exec operands

Every other *_exec insn has the exec operand last.  This being the other way
around is a cause of bugs, and prevents use in macro templates.

2020-04-23  Andrew Stubbs  <ams@codesourcery.com>

	gcc/
	* config/gcn/gcn-valu.md (mov<mode>_exec): Swap the numbers on operands
	2 and 3.
	(mov<mode>_exec): Likewise.
	(trunc<vndi><mode>2_exec): Swap parameters to gen_mov<mode>_exec.
	(<convop><mode><vndi>2_exec): Likewise.