[X86] Fast-ISel was incorrectly always killing the source of a truncate.

A trunc from i32 to i1 on x86_64 generates an instruction such as

%vreg19<def> = COPY %vreg9:sub_8bit<kill>; GR8:%vreg19 GR32:%vreg9

However, the copy here should only have the kill flag on the 32-bit path, not the 64-bit one.
Otherwise, we are killing the source of the truncate which could be used later in the program.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@236890 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Target/X86/X86FastISel.cpp

@@ -2185,6 +2185,7 @@ bool X86FastISel::X86SelectTrunc(const Instruction *I) {
     return true;
   }
 
+  bool KillInputReg = false;
   if (!Subtarget->is64Bit()) {
     // If we're on x86-32; we can't extract an i8 from a general register.
     // First issue a copy to GR16_ABCD or GR32_ABCD.
@@ -2194,11 +2195,12 @@ bool X86FastISel::X86SelectTrunc(const Instruction *I) {
     BuildMI(*FuncInfo.MBB, FuncInfo.InsertPt, DbgLoc,
             TII.get(TargetOpcode::COPY), CopyReg).addReg(InputReg);
     InputReg = CopyReg;
+    KillInputReg = true;
   }
 
   // Issue an extract_subreg.
   unsigned ResultReg = fastEmitInst_extractsubreg(MVT::i8,
-                                                  InputReg, /*Kill=*/true,
+                                                  InputReg, KillInputReg,
                                                   X86::sub_8bit);
   if (!ResultReg)
     return false;