Fix the test VPCMPESTRM_0x45_128 so that it doesn't depend on random junk in RAX...

Fix the test VPCMPESTRM_0x45_128 so that it doesn't depend on random junk in RAX and RDX.

This test fails sporadically (eg, on Fedora 27 with gcc (GCC) 7.3.1 20180303 (Red Hat 7.3.1-5))
because the tested instruction uses RAX and RDX as input, but the test framework doesn't
set them :-/, so the outcome basically depends on whatever junk the compiler left in those
registers beforehand.  As a result of this, all previous uses of RAX in the test have
been changed to use RSI instead.  n-i-bz.