[Thumb-1] Fix corner cases for compressed jump tables

Summary:
When synthesized TBB/TBH is expanded, we need to avoid the case of:
   BaseReg is redefined after the load of branching target. E.g.:

    %R2 = tLEApcrelJT <jt#1>
    %R1 =  tLDRr %R1, %R2    ==> %R2 = tLEApcrelJT <jt#1>
    %R2 = tLDRspi %SP, 12        %R2 = tLDRspi %SP, 12
    tBR_JTr %R1                  tTBB_JT %R2, %R1
`
Reviewers: jmolloy

Reviewed By: jmolloy

Subscribers: llvm-commits, rengolin

Differential Revision: https://reviews.llvm.org/D32250

llvm-svn: 300870

llvm/lib/Target/ARM/ARMConstantIslandPass.cpp

@@ -2157,6 +2157,15 @@ bool ARMConstantIslands::optimizeThumb2JumpTables() {
 
       // If we're in PIC mode, there should be another ADD following.
       auto *TRI = STI->getRegisterInfo();
+
+      // %base cannot be redefined after the load as it will appear before
+      // TBB/TBH like:
+      //      %base =
+      //      %base =
+      //      tBB %base, %idx
+      if (registerDefinedBetween(BaseReg, Load->getNextNode(), MBB->end(), TRI))
+        continue;
+
       if (isPositionIndependentOrROPI) {
         MachineInstr *Add = Load->getNextNode();
         if (Add->getOpcode() != ARM::tADDrr ||