i965/fs: Reset the register file to VGRF in lower_integer_multiplication

i965/fs: Reset the register file to VGRF in lower_integer_multiplication

18fde36ced4279f2577097a1a7d31b55f2f5f141 changed the way temporary
registers were allocated in lower_integer_multiplication so that we
allocate regs_written(inst) space and keep the stride of the original
destination register.  This was to ensure that any MUL which originally
followed the CHV/BXT integer multiply regioning restrictions would
continue to follow those restrictions even after lowering.  This works
fine except that I forgot to reset the register file to VGRF so, even
though they were assigned a number from alloc.allocate(), they had the
wrong register file.  This caused some GLES 3.0 CTS tests to start
failing on Sandy Bridge due to attempted reads from the MRF:

    ES3-CTS.functional.shaders.precision.int.highp_mul_fragment.snbm64
    ES3-CTS.functional.shaders.precision.int.mediump_mul_fragment.snbm64
    ES3-CTS.functional.shaders.precision.int.lowp_mul_fragment.snbm64
    ES3-CTS.functional.shaders.precision.uint.highp_mul_fragment.snbm64
    ES3-CTS.functional.shaders.precision.uint.mediump_mul_fragment.snbm64
    ES3-CTS.functional.shaders.precision.uint.lowp_mul_fragment.snbm64

This commit remedies this problem by, instead of copying inst->dst and
overwriting nr, just make a new register and set the region to match
inst->dst.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103626
Fixes: 18fde36ced4279f2577097a1a7d31b55f2f5f141
Cc: "17.3" <mesa-stable@lists.freedesktop.org>
Reviewed-by: Matt Turner <mattst88@gmail.com>