[AArch64][FastISel] Fix kill flags for integer extends.

In the case we optimize an integer extend away and replace it directly with the
source register, we also have to clear all kill flags at all its uses.
This is necessary, because the orignal IR instruction might be trivially dead,
but we replaced it with a nop at MI level.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@221628 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/AArch64/AArch64FastISel.cpp

@@ -4197,6 +4197,14 @@ bool AArch64FastISel::selectIntExt(const Instruction *I) {
             .addImm(AArch64::sub_32);
         SrcReg = ResultReg;
       }
+      // Conservatively clear all kill flags from all uses, because we are
+      // replacing a sign-/zero-extend instruction at IR level with a nop at MI
+      // level. The result of the instruction at IR level might have been
+      // trivially dead, which is now not longer true.
+      unsigned UseReg = lookUpRegForValue(I);
+      if (UseReg)
+        MRI.clearKillFlags(UseReg);
+
       updateValueMap(I, SrcReg);
       return true;
     }

test/CodeGen/AArch64/fast-isel-int-ext4.ll

@@ -0,0 +1,20 @@
+; RUN: llc -mtriple=aarch64-apple-darwin -fast-isel -fast-isel-abort -verify-machineinstrs < %s | FileCheck %s
+
+define i32 @kill_flag(i16 signext %a) {
+; CHECK-LABEL: kill_flag
+entry:
+  %0 = sext i16 %a to i32
+  br label %bb1
+
+bb1:
+  %1 = icmp slt i32 undef, %0
+  br i1 %1, label %loop, label %exit
+
+loop:
+  %2 = sext i16 %a to i32
+  %3 = icmp slt i32 undef, %2
+  br i1 %3, label %bb1, label %exit
+
+exit:
+  ret i32 0
+}