[GISel]: Add helpers for easy building G_FCONSTANT along with matchers

Added helpers to build G_FCONSTANT, along with matching ConstantFP and
unit tests for the same.

Sample usage.

auto MIB = Builder.buildFConstant(s32, 0.5); // Build IEEESingle
For Matching the above

const ConstantFP* Tmp;
mi_match(DstReg, MRI, m_GFCst(Tmp));

https://reviews.llvm.org/D44128
reviewed by: volkan

llvm-svn: 327152
diff --git a/llvm/lib/CodeGen/GlobalISel/Utils.cpp b/llvm/lib/CodeGen/GlobalISel/Utils.cpp
index b34414b..f39b387 100644
--- a/llvm/lib/CodeGen/GlobalISel/Utils.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/Utils.cpp
@@ -11,6 +11,7 @@
 //===----------------------------------------------------------------------===//
 
 #include "llvm/CodeGen/GlobalISel/Utils.h"
+#include "llvm/ADT/APFloat.h"
 #include "llvm/ADT/Twine.h"
 #include "llvm/CodeGen/GlobalISel/RegisterBankInfo.h"
 #include "llvm/CodeGen/MachineInstr.h"
@@ -212,3 +213,16 @@
   }
   return DefMI->getOpcode() == Opcode ? DefMI : nullptr;
 }
+
+APFloat llvm::getAPFloatFromSize(double Val, unsigned Size) {
+  if (Size == 32)
+    return APFloat(float(Val));
+  if (Size == 64)
+    return APFloat(Val);
+  if (Size != 16)
+    llvm_unreachable("Unsupported FPConstant size");
+  bool Ignored;
+  APFloat APF(Val);
+  APF.convert(APFloat::IEEEhalf(), APFloat::rmNearestTiesToEven, &Ignored);
+  return APF;
+}