[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/MachineIRBuilder.cpp b/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
index 475bb82..a6c285a 100644
--- a/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
+++ b/llvm/lib/CodeGen/GlobalISel/MachineIRBuilder.cpp
@@ -284,6 +284,14 @@
   return buildInstr(TargetOpcode::G_FCONSTANT).addDef(Res).addFPImm(&Val);
 }
 
+MachineInstrBuilder MachineIRBuilder::buildFConstant(unsigned Res, double Val) {
+  LLT DstTy = MRI->getType(Res);
+  auto &Ctx = MF->getFunction().getContext();
+  auto *CFP =
+      ConstantFP::get(Ctx, getAPFloatFromSize(Val, DstTy.getSizeInBits()));
+  return buildFConstant(Res, *CFP);
+}
+
 MachineInstrBuilder MachineIRBuilder::buildBrCond(unsigned Tst,
                                                   MachineBasicBlock &Dest) {
   assert(MRI->getType(Tst).isScalar() && "invalid operand type");