JitArm64: MultiplyImmediate - Handle 2^n

Turn multiplications by a power of two into bitshifts.

Before:
0x52800817   mov    w23, #0x40
0x1b167ef6   mul    w22, w23, w22

After:
0x531a66d6   lsl    w22, w22, #6

Source/Core/Core/PowerPC/JitArm64/JitArm64_Integer.cpp

@@ -7,6 +7,7 @@
 #include "Common/Assert.h"
 #include "Common/BitUtils.h"
 #include "Common/CommonTypes.h"
+#include "Common/MathUtil.h"
 
 #include "Core/Core.h"
 #include "Core/CoreTiming.h"
@@ -894,6 +895,15 @@ bool JitArm64::MultiplyImmediate(u32 imm, int a, int d, bool rc)
     if (rc)
       ComputeRC0(gpr.R(d));
   }
+  else if (MathUtil::IsPow2(imm))
+  {
+    const int shift = IntLog2(imm);
+
+    gpr.BindToRegister(d, d == a);
+    LSL(gpr.R(d), gpr.R(a), shift);
+    if (rc)
+      ComputeRC0(gpr.R(d));
+  }
   else
   {
     // Immediate did not match any known special cases.