Properly handle EVEX register aliases

gas/

	* config/tc-i386.c (parse_register): Set need_vrex.

gas/testsuite/

	* gas/i386/x86-64-equ.d: New.
	* gas/i386/x86-64-equ.s: New.
	* gas/i386/i386.exp: Run x86-64-equ.
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