[AMDGPU] Forward-declare MachineIRBuilder (NFC)

AMDGPULegalizerInfo.h needs MachineIRBuilder but relies on a forward
declaration of MachineIRBuilder in LegalizerInfo.h.  This patch adds a
forward declaration right in AMDGPULegalizerInfo.h.

While we are at it, this patch removes the one in LegalizerInfo.h,
where it is unnecessary.

Author: kazutakahirata

llvm/include/llvm/CodeGen/GlobalISel/LegalizerInfo.h

@@ -37,7 +37,6 @@ extern cl::opt<bool> DisableGISelLegalityCheck;
 
 class LegalizerHelper;
 class MachineInstr;
-class MachineIRBuilder;
 class MachineRegisterInfo;
 class MCInstrInfo;
 class GISelChangeObserver;

llvm/lib/Target/AMDGPU/AMDGPULegalizerInfo.h

@@ -23,6 +23,7 @@ namespace llvm {
 class GCNTargetMachine;
 class LLVMContext;
 class GCNSubtarget;
+class MachineIRBuilder;
 
 namespace AMDGPU {
 struct ImageDimIntrinsicInfo;