Jit64: divwx - Micro-optimize certain divisors

When the multiplier is positive (which is the most common case), we can
generate slightly better code.

- Division by 30307
Before:
49 63 C5             movsxd      rax,r13d
48 69 C0 65 6B 32 45 imul        rax,rax,45326B65h
4C 8B C0             mov         r8,rax
48 C1 E8 3F          shr         rax,3Fh
49 C1 F8 2D          sar         r8,2Dh
44 03 C0             add         r8d,eax

After:
49 63 C5             movsxd      rax,r13d
4C 69 C0 65 6B 32 45 imul        r8,rax,45326B65h
C1 E8 1F             shr         eax,1Fh
49 C1 F8 2D          sar         r8,2Dh
44 03 C0             add         r8d,eax

Source/Core/Core/PowerPC/Jit64/Jit_Integer.cpp

@@ -1484,6 +1484,12 @@ void Jit64::divwx(UGeckoInstruction inst)
         SHR(32, Rd, Imm8(31));
         SAR(32, R(RSCRATCH), Imm8(m.shift));
       }
+      else if (m.multiplier > 0)
+      {
+        IMUL(64, Rd, R(RSCRATCH), Imm32(m.multiplier));
+        SHR(32, R(RSCRATCH), Imm8(31));
+        SAR(64, R(Rd), Imm8(32 + m.shift));
+      }
       else
       {
         IMUL(64, RSCRATCH, R(RSCRATCH), Imm32(m.multiplier));