Aarch64: fix shift ops: mask shift amount.

The failure to mask the amount triggered a panic due to a subtraction
overflow check; see
https://bugzilla.mozilla.org/show_bug.cgi?id=1649432. Attempting to
shift by an out-of-range amount should be defined to shift by an amount
mod the operand size (i.e., masked to 5 bits for 32-bit shifts, or 6
bits for 64-bit shifts).

cranelift/filetests/filetests/vcode/aarch64/shift-op.clif

@@ -15,3 +15,17 @@ block0(v0: i64):
 ; nextln: mov sp, fp
 ; nextln: ldp fp, lr, [sp], #16
 ; nextln: ret
+
+function %f(i32) -> i32 {
+block0(v0: i32):
+  v1 = iconst.i32 53
+  v2 = ishl.i32 v0, v1
+  return v2
+}
+
+; check: stp fp, lr, [sp, #-16]!
+; nextln: mov fp, sp
+; nextln: lsl w0, w0, #21
+; nextln: mov sp, fp
+; nextln: ldp fp, lr, [sp], #16
+; nextln: ret