GlobalISel: Fix insert point in CSEMIRBuilder unit test

[arsenm]

Summary

This was using invalid MIR for the test instructions. The test add was
the first instruction in the block, before the trunc inputs or copies
from physical registers which I assume was not intended.

[aditya_nandakumar]

Silly bug. Thanks for fixing.

[arsenm]

bdad24c2cc9f759a974cb63aaa3fff62b978726d

llvm/unittests/CodeGen/GlobalISel/CSETest.cpp

TEST_F(AArch64GISelMITest, TestCSE) {
   auto MIBInput1 = B.buildInstr(TargetOpcode::G_TRUNC, {s16}, {Copies[1]});
   auto MIBAdd = B.buildInstr(TargetOpcode::G_ADD, {s16}, {MIBInput, MIBInput});
   GISelCSEInfo CSEInfo;
   CSEInfo.setCSEConfig(std::make_unique<CSEConfigFull>());
   CSEInfo.analyze(*MF);
   B.setCSEInfo(&CSEInfo);
   CSEMIRBuilder CSEB(B.getState());
 
-  CSEB.setInsertPt(*EntryMBB, EntryMBB->begin());
-  unsigned AddReg = MRI->createGenericVirtualRegister(s16);
+  CSEB.setInsertPt(B.getMBB(), B.getInsertPt());
+  Register AddReg = MRI->createGenericVirtualRegister(s16);
   auto MIBAddCopy =
       CSEB.buildInstr(TargetOpcode::G_ADD, {AddReg}, {MIBInput, MIBInput});
   EXPECT_EQ(MIBAddCopy->getOpcode(), TargetOpcode::COPY);
   auto MIBAdd2 =
       CSEB.buildInstr(TargetOpcode::G_ADD, {s16}, {MIBInput, MIBInput});
   EXPECT_TRUE(&*MIBAdd == &*MIBAdd2);
   auto MIBAdd4 =
       CSEB.buildInstr(TargetOpcode::G_ADD, {s16}, {MIBInput, MIBInput});