[X86] Use correct type for return value of ComputeAvailableFeatures in the AsmParser. NFC

There aren't enough used bits to make this a functional change, but we should fix it for consistency.

llvm-svn: 316639
diff --git a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
index 0d211c4..896c50a 100644
--- a/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
+++ b/llvm/lib/Target/X86/AsmParser/X86AsmParser.cpp
@@ -909,7 +909,7 @@
     MCSubtargetInfo &STI = copySTI();
     FeatureBitset AllModes({X86::Mode64Bit, X86::Mode32Bit, X86::Mode16Bit});
     FeatureBitset OldMode = STI.getFeatureBits() & AllModes;
-    unsigned FB = ComputeAvailableFeatures(
+    uint64_t FB = ComputeAvailableFeatures(
       STI.ToggleFeature(OldMode.flip(mode)));
     setAvailableFeatures(FB);