i386: Also check R12-R15 registers when optimizing testq to testb

i386: Also check R12-R15 registers when optimizing testq to testb

Similar to SP, BP, SI and DI registers, R12-R15 registers must use REX
prefix for the low byte register when optimizing

test $imm7, %r64/%r32/%r16 -> test $imm7, %r8

	PR gas/25274
	* config/tc-i386.c (optimize_encoding): Also check R12-R15
	registers for "test $imm7, %r64/%r32/%r16 -> test $imm7, %r8"
	optimization.
	* testsuite/gas/i386/x86-64-optimize-3.s: Add tests for test
	with r12.
	* testsuite/gas/i386/x86-64-optimize-3.d: Updated.
	* testsuite/gas/i386/x86-64-optimize-3b.d: Likewise.