Merge lowerCall and lowerRet between x86 and x64

src/IceTargetLoweringX8664Traits.h

Index: src/IceTargetLoweringX8664Traits.h
diff --git a/src/IceTargetLoweringX8664Traits.h b/src/IceTargetLoweringX8664Traits.h
index b206817eea4a413744963eade01f362ba788425d..5df51f5a5bd000633b98022cb9bee1230809a4dc 100644
--- a/src/IceTargetLoweringX8664Traits.h
+++ b/src/IceTargetLoweringX8664Traits.h
@@ -720,10 +720,47 @@ public:
 
   static int32_t getRdxOrDie() { return RegisterSet::Reg_rdx; }
 
+  // x86-64 calling convention:
+  //
+  // * The first eight arguments of vector/fp type, regardless of their
+  // position relative to the other arguments in the argument list, are placed
+  // in registers %xmm0 - %xmm7.
+  //
+  // * The first six arguments of integer types, regardless of their position
+  // relative to the other arguments in the argument list, are placed in
+  // registers %rdi, %rsi, %rdx, %rcx, %r8, and %r9.
+  //
+  // This intends to match the section "Function Calling Sequence" of the
+  // document "System V Application Binary Interface."
+
   /// The maximum number of arguments to pass in XMM registers
   static const uint32_t X86_MAX_XMM_ARGS = 8;
   /// The maximum number of arguments to pass in GPR registers
   static const uint32_t X86_MAX_GPR_ARGS = 6;
+  /// Whether scalar floating point arguments are passed in XMM registers
+  static const bool X86_PASS_SCALAR_FP_IN_XMM = true;
+  /// Get the register for a given argument slot in the XMM registers.
+  static inline int32_t getRegisterForXmmArgNum(uint32_t ArgNum) {
+    if (ArgNum >= X86_MAX_XMM_ARGS) {
+      return Variable::NoRegister;
+    }
+    return static_cast<int32_t>(RegisterSet::Reg_xmm0 + ArgNum);
+  }
+  /// Get the register for a given argument slot in the GPRs.
+  static inline int32_t getRegisterForGprArgNum(Type Ty, uint32_t ArgNum) {
+    if (ArgNum >= X86_MAX_GPR_ARGS) {
+      return Variable::NoRegister;
+    }
+    static const RegisterSet::AllRegisters GprForArgNum[] = {
+        RegisterSet::Reg_rdi, RegisterSet::Reg_rsi, RegisterSet::Reg_rdx,
+        RegisterSet::Reg_rcx, RegisterSet::Reg_r8,  RegisterSet::Reg_r9,
+    };
+    static_assert(llvm::array_lengthof(GprForArgNum) == X86_MAX_GPR_ARGS,
+                  "Mismatch between MAX_GPR_ARGS and GprForArgNum.");
+    assert(Ty == IceType_i64 || Ty == IceType_i32);
+    return static_cast<int32_t>(getGprForType(Ty, GprForArgNum[ArgNum]));
+  }
+
   /// The number of bits in a byte
   static const uint32_t X86_CHAR_BIT = 8;
   /// Stack alignment. This is defined in IceTargetLoweringX8664.cpp because it