[X86] Cleanup 'x' and 'y' mnemonic suffixes for vcvtpd2dq/vcvttpd2dq/vcvtpd2ps...

[X86] Cleanup 'x' and 'y' mnemonic suffixes for vcvtpd2dq/vcvttpd2dq/vcvtpd2ps and similar instructions.

-Don't print the 'x' suffix for the 128-bit reg/mem VEX encoded instructions in Intel syntax. This is consistent with the EVEX versions.
-Don't print the 'y' suffix for the 256-bit reg/reg VEX encoded instructions in Intel or AT&T syntax. This is consistent with the EVEX versions.
-Allow the 'x' and 'y' suffixes to be used for the reg/mem forms when we're assembling using Intel syntax.
-Allow the 'x' and 'y' suffixes on the reg/reg EVEX encoded instructions in Intel or AT&T syntax. This is consistent with what VEX was already allowing.

This should fix at least some of PR28850.

llvm-svn: 286787