[amdgcn] Use first lane of v1 for zero offset

[amdgcn] Use first lane of v1 for zero offset

Use v1 instead of v0 when a zero-valued VGPR is needed.  This frees up
v0 for other purposes.

2019-11-15  Kwok Cheung Yeung  <kcy@codesourcery.com>

	gcc/
	* config/gcn/gcn.c (gcn_expand_prologue): Remove initialization and
	prologue use of v0.
	(print_operand_address): Use v1 for zero vector offset.

From-SVN: r278297