VAX: Add the `movmemhi' instruction

VAX: Add the `movmemhi' instruction

The MOVC3 machine instruction has `memmove' semantics[1]:

"The operation of the instruction is such that overlap of the source and
destination strings does not affect the result."

so use it to provide the `movmemhi' instruction as well.

References:

[1] DEC STD 032-0 "VAX Architecture Standard", Digital Equipment
    Corporation, A-DS-EL-00032-00-0 Rev J, December 15, 1989, Section
    3.10 "Character-String Instructions", p. 3-162

	gcc/
	* config/vax/vax.md (cpymemhi1): Rename insn to...
	(movmemhi1): ... this.
	(cpymemhi): Update accordingly.  Remove constraints.
	(movmemhi): New expander.

	gcc/testsuite/
	* gcc.target/vax/movmem.c: New test.