InstSimplify: Simplify trivial pointer expressions like b + (e - b)

consider:
long long *f(long long *b, long long *e) {
  return b + (e - b);
}

we would lower this to something like:
define i64* @f(i64* %b, i64* %e) {
  %1 = ptrtoint i64* %e to i64
  %2 = ptrtoint i64* %b to i64
  %3 = sub i64 %1, %2
  %4 = ashr exact i64 %3, 3
  %5 = getelementptr inbounds i64* %b, i64 %4
  ret i64* %5
}

This should fold away to just 'e'.

N.B.  This adds m_SpecificInt as a convenient way to match against a
particular 64-bit integer when using LLVM's match interface.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216439 91177308-0d34-0410-b5e6-96231b3b80d8

test/Transforms/InstSimplify/gep.ll

@@ -0,0 +1,37 @@
+; RUN: opt -S -instsimplify < %s | FileCheck %s
+
+target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"
+
+%struct.A = type { [7 x i8] }
+
+define %struct.A* @test1(%struct.A* %b, %struct.A* %e) {
+  %e_ptr = ptrtoint %struct.A* %e to i64
+  %b_ptr = ptrtoint %struct.A* %b to i64
+  %sub = sub i64 %e_ptr, %b_ptr
+  %sdiv = sdiv exact i64 %sub, 7
+  %gep = getelementptr inbounds %struct.A* %b, i64 %sdiv
+  ret %struct.A* %gep
+; CHECK-LABEL: @test1
+; CHECK-NEXT: ret %struct.A* %e
+}
+
+define i8* @test2(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: @test2
+; CHECK-NEXT: ret i8* %e
+}
+
+define i64* @test3(i64* %b, i64* %e) {
+  %e_ptr = ptrtoint i64* %e to i64
+  %b_ptr = ptrtoint i64* %b to i64
+  %sub = sub i64 %e_ptr, %b_ptr
+  %ashr = ashr exact i64 %sub, 3
+  %gep = getelementptr inbounds i64* %b, i64 %ashr
+  ret i64* %gep
+; CHECK-LABEL: @test3
+; CHECK-NEXT: ret i64* %e
+}