Eliminate useless insertelement instructions. This implements

Transforms/InstCombine/vec_insertelt.ll and fixes PR1286.

We now compile the code from that bug into:

_foo:
        movl 4(%esp), %eax
        movdqa (%eax), %xmm0
        movl 8(%esp), %ecx
        psllw (%ecx), %xmm0
        movdqa %xmm0, (%eax)
        ret

instead of:

_foo:
        subl $4, %esp
        movl %ebp, (%esp)
        movl %esp, %ebp
        movl 12(%ebp), %eax
        movdqa (%eax), %xmm0
        #IMPLICIT_DEF %eax
        pinsrw $2, %eax, %xmm0
        xorl %ecx, %ecx
        pinsrw $3, %ecx, %xmm0
        pinsrw $4, %eax, %xmm0
        pinsrw $5, %ecx, %xmm0
        pinsrw $6, %eax, %xmm0
        pinsrw $7, %ecx, %xmm0
        movl 8(%ebp), %eax
        movdqa (%eax), %xmm1
        psllw %xmm0, %xmm1
        movdqa %xmm1, (%eax)
        movl %ebp, %esp
        popl %ebp
        ret

woo :)

llvm-svn: 35788

llvm/lib/Transforms/Scalar/InstructionCombining.cpp

@@ -9160,6 +9160,10 @@ Instruction *InstCombiner::visitInsertElementInst(InsertElementInst &IE) {
   Value *ScalarOp = IE.getOperand(1);
   Value *IdxOp    = IE.getOperand(2);
   
+  // Inserting an undef or into an undefined place, remove this.
+  if (isa<UndefValue>(ScalarOp) || isa<UndefValue>(IdxOp))
+    ReplaceInstUsesWith(IE, VecOp);
+  
   // If the inserted element was extracted from some other vector, and if the 
   // indexes are constant, try to turn this into a shufflevector operation.
   if (ExtractElementInst *EI = dyn_cast<ExtractElementInst>(ScalarOp)) {