[AArch64][Inline-Asm] Return the 32-bit floating point register class

when constraint "w" is used on a 32-bit operand.

This enables compiling the following code, which used to error out in
the backend:

void foo1(int a) {
  asm volatile ("sqxtn h0, %s0\n" : : "w"(a):);
}

Fixes PR28633.

llvm-svn: 276344