amdgcn: fix vcc clobber in vector load/store

amdgcn: fix vcc clobber in vector load/store

This switches the code that expands scalar addresses to vectors of addresses
from using VCC to using CC_SAVE_REG, for the lo-part to hi-part carry values.
These were fine in code expanded in earlier passes, but addresses expanded
late, such as for stack spills or reloads, could clobber live VCC values,
causing execution failures.

This is the first target-specific testcase for GCN, so the new .exp file is
included.

Backport from master:

	gcc/
	* config/gcn/gcn-valu.md (add<mode>3_zext_dup): Change to a
	define_expand, and rename the original to ...
	(add<mode>3_vcc_zext_dup): ... this, and add a custom VCC operand.
	(add<mode>3_zext_dup_exec): Likewise, with ...
	(add<mode>3_vcc_zext_dup_exec): ... this.
	(add<mode>3_zext_dup2): Likewise, with ...
	(add<mode>3_zext_dup_exec): ... this.
	(add<mode>3_zext_dup2_exec): Likewise, with ...
	(add<mode>3_zext_dup2): ... this.
	* config/gcn/gcn.c (gcn_expand_scalar_to_vector_address): Switch
	addv64di3_zext* calls to use addv64di3_vcc_zext*.

	gcc/testsuite/
	* gcc.target/gcn/gcn.exp: New file.
	* gcc.target/gcn/vcc-clobber.c: New file.