Index: lib/Transforms/InstCombine/InstructionCombining.cpp
===================================================================
--- lib/Transforms/InstCombine/InstructionCombining.cpp
+++ lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -150,6 +150,12 @@
   bool FromLegal = FromWidth == 1 || DL.isLegalInteger(FromWidth);
   bool ToLegal = ToWidth == 1 || DL.isLegalInteger(ToWidth);
 
+  // If the type to convert to is i8 or i16 (a very common case), then
+  // we convert whether it is a legal type or not. This can often open up
+  // extra combine opportunities.
+  if (FromLegal && ToWidth < FromWidth && (ToWidth == 8 || ToWidth == 16))
+    return true;
+
   // If this is a legal integer from type, and the result would be an illegal
   // type, don't do the transformation.
   if (FromLegal && !ToLegal)
Index: test/Transforms/InstCombine/should-change-type.ll
===================================================================
--- /dev/null
+++ test/Transforms/InstCombine/should-change-type.ll
@@ -0,0 +1,45 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py
+; RUN: opt < %s -instcombine -S | FileCheck %s
+target datalayout = "e-m:e-p:32:32-i64:64-v128:64:128-a:0:32-n32-S64"
+
+; Tests for removing zext/trunk from/to i8 and i16, even if it is not a
+; legal type.
+
+define i8 @test1(i8 %x, i8 %y) {
+; CHECK-LABEL: @test1(
+; CHECK-NEXT:    [[C:%.*]] = add i8 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i8 [[C]]
+;
+  %xz = zext i8 %x to i32
+  %yz = zext i8 %y to i32
+  %c = add i32 %xz, %yz
+  %d = trunc i32 %c to i8
+  ret i8 %d
+}
+
+define i16 @test2(i16 %x, i16 %y) {
+; CHECK-LABEL: @test2(
+; CHECK-NEXT:    [[C:%.*]] = add i16 [[X:%.*]], [[Y:%.*]]
+; CHECK-NEXT:    ret i16 [[C]]
+;
+  %xz = zext i16 %x to i32
+  %yz = zext i16 %y to i32
+  %c = add i32 %xz, %yz
+  %d = trunc i32 %c to i16
+  ret i16 %d
+}
+
+define i9 @test3(i9 %x, i9 %y) {
+; CHECK-LABEL: @test3(
+; CHECK-NEXT:    [[XZ:%.*]] = zext i9 [[X:%.*]] to i32
+; CHECK-NEXT:    [[YZ:%.*]] = zext i9 [[Y:%.*]] to i32
+; CHECK-NEXT:    [[C:%.*]] = add nuw nsw i32 [[XZ]], [[YZ]]
+; CHECK-NEXT:    [[D:%.*]] = trunc i32 [[C]] to i9
+; CHECK-NEXT:    ret i9 [[D]]
+;
+  %xz = zext i9 %x to i32
+  %yz = zext i9 %y to i32
+  %c = add i32 %xz, %yz
+  %d = trunc i32 %c to i9
+  ret i9 %d
+}