i965/vec4: Only examine virtual_grf_end for GRF sources

i965/vec4: Only examine virtual_grf_end for GRF sources

If the source is not a GRF, it could have a register >= virtual_grf_count.
Accessing virtual_grf_end with such a register would lead to
out-of-bounds access.  Make sure the source is a GRF before accessing
virtual_grf_end.

Fixes Valgrind complaints while compiling some shaders.

Signed-off-by: Ian Romanick <ian.d.romanick@intel.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>
Cc: mesa-stable@lists.freedesktop.org