Jit64AsmCommon: Micro-optimize GenFrsqrte

Save 2 bytes by testing register against itself and branching on the 
sign flag.

Before:
0:  48 0f ba e0 3f          bt     rax,0x3f

After:
0:  48 85 c0                test   rax,rax

Source/Core/Core/PowerPC/Jit64Common/Jit64AsmCommon.cpp

@@ -101,8 +101,8 @@ void CommonAsmRoutines::GenFrsqrte()
   MOVQ_xmm(XMM0, R(RSCRATCH));
   RET();
   SetJumpTarget(inf);
-  BT(64, R(RSCRATCH), Imm8(63));
-  FixupBranch negative = J_CC(CC_C);
+  TEST(64, R(RSCRATCH), R(RSCRATCH));
+  FixupBranch negative = J_CC(CC_S);
   XORPD(XMM0, R(XMM0));
   RET();