Reland "[Assembler][x64] Make immediates immutable"

This is a reland of 01db326c.

Original change's description:
> [Assembler][x64] Make immediates immutable
> 
> On x64, we already pass immediates by value. This CL ensures that this
> is indeed cheap, and it makes immediates immutable.
> 
> R=mstarzinger@chromium.org
> 
> Bug: v8:7310
> Change-Id: I53a0666d53b9de69d390621298798c03b5190497
> Reviewed-on: https://chromium-review.googlesource.com/934341
> Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
> Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#51613}

Bug: v8:7310
Change-Id: I37f1bd1528c742e8df11b7f524316935dd289f00
Reviewed-on: https://chromium-review.googlesource.com/941621Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#51655}

src/x64/assembler-x64.h

@@ -301,23 +301,25 @@ enum RoundingMode {
 // -----------------------------------------------------------------------------
 // Machine instruction Immediates
 
-class Immediate BASE_EMBEDDED {
+class Immediate {
  public:
-  explicit Immediate(int32_t value) : value_(value) {}
-  explicit Immediate(int32_t value, RelocInfo::Mode rmode)
+  explicit constexpr Immediate(int32_t value) : value_(value) {}
+  explicit constexpr Immediate(int32_t value, RelocInfo::Mode rmode)
       : value_(value), rmode_(rmode) {}
-  explicit Immediate(Smi* value) {
+  explicit Immediate(Smi* value)
+      : value_(static_cast<int32_t>(reinterpret_cast<intptr_t>(value))) {
     DCHECK(SmiValuesAre31Bits());  // Only available for 31-bit SMI.
-    value_ = static_cast<int32_t>(reinterpret_cast<intptr_t>(value));
   }
 
  private:
-  int32_t value_;
-  RelocInfo::Mode rmode_ = RelocInfo::NONE;
+  const int32_t value_;
+  const RelocInfo::Mode rmode_ = RelocInfo::NONE;
 
   friend class Assembler;
 };
-
+ASSERT_TRIVIALLY_COPYABLE(Immediate);
+static_assert(sizeof(Immediate) <= kPointerSize,
+              "Immediate must be small enough to pass it by value");
 
 // -----------------------------------------------------------------------------
 // Machine instruction Operands