Revert "[AArch64][GlobalISel] Legalize bswap <2 x i16>"

This reverts commit 5cd63e9e.

https://bugs.llvm.org/show_bug.cgi?id=51707

The sequence feeding in/out of the rev32/ushr isn't quite right:

 _swap:
         ldr     h0, [x0]
         ldr     h1, [x0, #2]
-        mov     v0.h[1], v1.h[0]
+        mov     v0.s[1], v1.s[0]
         rev32   v0.8b, v0.8b
         ushr    v0.2s, v0.2s, #16
-        mov     h1, v0.h[1]
+        mov     s1, v0.s[1]
         str     h0, [x0]
         str     h1, [x0, #2]
         ret