[AArch64] Generate rev16/rev32 from bswap + srl when upper bits are known zero.

Canonicalize (srl (bswap i32 x), 16) to (rotr (bswap i32 x), 16), if the high
16-bits of x are zero. Similarly, canonicalize (srl (bswap i64 x), 32) to
(rotr (bswap i64 x), 32), if the high 32-bits of x are zero.

test_rev_w_srl16:            test_rev_w_srl16:
  and w8, w0, #0xffff          and     w8, w0, #0xffff
  rev w8, w8           --->    rev16   w0, w8
  lsr     w0, w8, #16

test_rev_x_srl32:            test_rev_x_srl32:
  rev x8, x8           --->    rev32   x0, x8
  lsr x0, x8, #32

llvm-svn: 270896