[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@316639 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed