ARM: Canonicalize access to function attributes, NFC

ARM: Canonicalize access to function attributes, NFC

Canonicalize access to function attributes to use the simpler API.

getAttributes().getAttribute(AttributeSet::FunctionIndex, Kind)
  => getFnAttribute(Kind)

getAttributes().hasAttribute(AttributeSet::FunctionIndex, Kind)
  => hasFnAttribute(Kind)

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229220 91177308-0d34-0410-b5e6-96231b3b80d8