InstSimplify: Don't simplify gep X, (Y-X) to Y if types differ

It's incorrect to perform this simplification if the types differ.
A bitcast would need to be inserted for this to work.

This fixes PR20771.

llvm-svn: 216597
diff --git a/llvm/test/Transforms/InstSimplify/gep.ll b/llvm/test/Transforms/InstSimplify/gep.ll
index d1704bf..49a97f1 100644
--- a/llvm/test/Transforms/InstSimplify/gep.ll
+++ b/llvm/test/Transforms/InstSimplify/gep.ll
@@ -64,3 +64,17 @@
 ; CHECK-LABEL: @test6
 ; CHECK-NEXT: ret i64* null
 }
+
+define i8* @test7(i8* %b, i8** %e) {
+  %e_ptr = ptrtoint i8** %e to i64
+  %b_ptr = ptrtoint i8* %b to i64
+  %sub = sub i64 %e_ptr, %b_ptr
+  %gep = getelementptr inbounds i8* %b, i64 %sub
+  ret i8* %gep
+; CHECK-LABEL: @test7
+; CHECK-NEXT: ptrtoint
+; CHECK-NEXT: ptrtoint
+; CHECK-NEXT: sub
+; CHECK-NEXT: getelementptr
+; CHECK-NEXT: ret
+}