PPC: [turbofan] Introduce optional Float64Min and Float64Max machine operators.

Port 99f8d57f

Original commit message:
Basically recognize certain x < y ? x : y constructs and turn that into
Float64Min/Float64Max operations, if the target machine supports that.
On x86 we lower to (v)minsd/(v)maxsd.

R=dcarney@chromium.org

Review URL: https://codereview.chromium.org/998283002

Cr-Commit-Position: refs/heads/master@{#27160}

R=danno@chromium.org, svenpanne@chromium.org

BUG=

Review URL: https://codereview.chromium.org/1005693002

Cr-Commit-Position: refs/heads/master@{#27174}

src/compiler/ppc/instruction-selector-ppc.cc

@@ -963,6 +963,12 @@ void InstructionSelector::VisitFloat64Mod(Node* node) {
 }
 
 
+void InstructionSelector::VisitFloat64Max(Node* node) { UNREACHABLE(); }
+
+
+void InstructionSelector::VisitFloat64Min(Node* node) { UNREACHABLE(); }
+
+
 void InstructionSelector::VisitFloat64Sqrt(Node* node) {
   VisitRRFloat64(this, kPPC_SqrtFloat64, node);
 }