Implement SqmulRoundSat for interpreter

Implemented `SqmulRoundSat` for the Cranelift interpreter, performing
QN-format fixed point multiplication for 16 and 32-bit integers in
SIMD vectors.

Copyright (c) 2021, Arm Limited

cranelift/filetests/filetests/runtests/simd-sqmulroundsat-aarch64.clif

@@ -0,0 +1,12 @@
+test interpret
+test run
+target aarch64
+;; x86_64 hasn't implemented this for `i32x4`
+
+function %sqmulrs_i32x4(i32x4, i32x4) -> i32x4 {
+block0(v0: i32x4, v1: i32x4):
+    v2 = sqmul_round_sat v0, v1
+    return v2
+}
+; run: %sqmulrs_i32x4([1000 2000 3000 4000], [10000 100000 1000000 10000000]) == [0 0 1 19]
+; run: %sqmulrs_i32x4([2147483647 -2147483648 -2147483648 0], [2147483647 -2147483648 2147483647 0]) == [2147483646 2147483647 -2147483647 0]