aarch64: Avoid bogus atomics match

The non-LSE pattern aarch64_atomic_exchange<mode> comes before the
LSE pattern aarch64_atomic_exchange<mode>_lse.  From a recog
perspective, the only difference between the patterns is that
the non-LSE one clobbers CC and needs a scratch.

However, combine and RTL-SSA can both add clobbers to make a
pattern match.  This means that if they try to rerecognise an
LSE pattern, they could end up turning it into a non-LSE pattern.
This patch adds a !TARGET_LSE test to avoid that.

This is needed to avoid a regression with later patches.

gcc/
	* config/aarch64/atomics.md (aarch64_atomic_exchange<mode>): Require
	!TARGET_LSE.

gcc/config/aarch64/atomics.md

@@ -224,7 +224,7 @@
       UNSPECV_ATOMIC_EXCHG))
    (clobber (reg:CC CC_REGNUM))
    (clobber (match_scratch:SI 4 "=&r"))]
-  ""
+  "!TARGET_LSE"
   "#"
   "&& epilogue_completed"
   [(const_int 0)]