AVX-512 instructions that use RIP-relative addressing and require the
R bit of the EVEX prefix to be zero, i.e., instructions that use Z8-Z15 or
Z24-Z31, are incorrectly encoded by the assembler.  The reason is that
the location of the offset at which the relative address is to be written
is incorrectly computed when the R bit is clear.

For example,

VMOVUPS bInitX<>+0(SB), Z0

encodes correctly to

62 f1 7c 48 10 05 66 e9 02 00

whereas

VMOVUPS bInitX<>+0(SB), Z8

encodes incorrectly to

62 71 7c 48 10 05 00 56 e9 02 00

Note the extra zero byte between the ModR/M byte (05) and the relative
address starting with 56.  This error results in the first byte of the
following instruction being overwritten and typically, a program crash.

This commit fixes the issue in the same way that is fixed for VEX encoded
instructions, by simply not incrementing the offset for EVEX instructions.
Existing test code created for a similar VEX encoding issue (19518) has
been modified to also test for the issue addressed by this commit.

Fixes #31001

Change-Id: If84719ac22ebb5fb3c42ff96cd32b611ad497414
Reviewed-on: https://go-review.googlesource.com/c/go/+/168562
Run-TryBot: Ilya Tocar <ilya.tocar@intel.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ilya Tocar <ilya.tocar@intel.com>