Remove a top-level "using namespace" directive from LegalizationArtifactCombiner.h

The directive pollutes the namespace of all files including that header.
Move the directive into the bodies of functions that need it instead.

Author: labath

llvm/include/llvm/CodeGen/GlobalISel/LegalizationArtifactCombiner.h

@@ -28,7 +28,6 @@
 #include "llvm/Support/Debug.h"
 
 #define DEBUG_TYPE "legalizer"
-using namespace llvm::MIPatternMatch;
 
 namespace llvm {
 class LegalizationArtifactCombiner {
@@ -57,6 +56,7 @@ class LegalizationArtifactCombiner {
                         SmallVectorImpl<MachineInstr *> &DeadInsts,
                         SmallVectorImpl<Register> &UpdatedDefs,
                         GISelObserverWrapper &Observer) {
+    using namespace llvm::MIPatternMatch;
     assert(MI.getOpcode() == TargetOpcode::G_ANYEXT);
 
     Builder.setInstrAndDebugLoc(MI);
@@ -110,6 +110,7 @@ class LegalizationArtifactCombiner {
                       SmallVectorImpl<MachineInstr *> &DeadInsts,
                       SmallVectorImpl<Register> &UpdatedDefs,
                       GISelObserverWrapper &Observer) {
+    using namespace llvm::MIPatternMatch;
     assert(MI.getOpcode() == TargetOpcode::G_ZEXT);
 
     Builder.setInstrAndDebugLoc(MI);
@@ -171,6 +172,7 @@ class LegalizationArtifactCombiner {
   bool tryCombineSExt(MachineInstr &MI,
                       SmallVectorImpl<MachineInstr *> &DeadInsts,
                       SmallVectorImpl<Register> &UpdatedDefs) {
+    using namespace llvm::MIPatternMatch;
     assert(MI.getOpcode() == TargetOpcode::G_SEXT);
 
     Builder.setInstrAndDebugLoc(MI);
@@ -228,6 +230,7 @@ class LegalizationArtifactCombiner {
                        SmallVectorImpl<MachineInstr *> &DeadInsts,
                        SmallVectorImpl<Register> &UpdatedDefs,
                        GISelObserverWrapper &Observer) {
+    using namespace llvm::MIPatternMatch;
     assert(MI.getOpcode() == TargetOpcode::G_TRUNC);
 
     Builder.setInstr(MI);
@@ -1282,6 +1285,8 @@ class LegalizationArtifactCombiner {
   /// Looks through copy instructions and returns the actual
   /// source register.
   Register lookThroughCopyInstrs(Register Reg) {
+    using namespace llvm::MIPatternMatch;
+
     Register TmpReg;
     while (mi_match(Reg, MRI, m_Copy(m_Reg(TmpReg)))) {
       if (MRI.getType(TmpReg).isValid())