[nvptx] Use '%' as register prefix

[nvptx] Use '%' as register prefix

The percentage sign as first character of a ptx identifier can be used to
avoid name conflicts, e.g., between user-defined variable names and
compiler-generated names.

The insn nvptx_uniform_warp_check contains register names without '%' prefix,
which potentially could lead to name conflicts with user-defined variable
names.

Fix this by adding a '%' prefix, more specifically a '%r_' prefix to avoid a
name conflict with ptx special registers.

Tested on x86_64 with nvptx accelerator.

gcc/ChangeLog:

2022-03-20  Tom de Vries  <tdevries@suse.de>

	PR target/104925
	* config/nvptx/nvptx.md (define_insn "nvptx_uniform_warp_check"):
	Use % as register prefix.