x86/{,V}AES: adjust when to force EVEX encoding

x86/{,V}AES: adjust when to force EVEX encoding

Commit a79d13a01f8c ("i386: Fix aes/vaes patterns [PR114576]") correctly
said "..., but we need to emit {evex} prefix in the assembly if AES ISA
is not enabled". Yet it did so only for the TARGET_AES insns. Going from
the alternative chosen in the TARGET_VAES insns isn't quite right: If
AES is (also) enabled, EVEX encoding would needlessly be forced.

gcc/

	* config/i386/sse.md (vaesdec_<mode>, vaesdeclast_<mode>,
	vaesenc_<mode>, vaesenclast_<mode>): Replace which_alternative
	check by TARGET_AES one.