make small (<= size_of::<int>()) enums immediate

[thestinger]

C-like enums are excluded from this for now, because the code paths
specific to them need to be changed.

fn foo() -> Option<~int> { Some(~5) }

Before:

; Function Attrs: uwtable
define void @_ZN3foo18hdec6e36682b87eeaf4v0.0E(%"enum.std::option::Option<~int>[#1]"* noalias nocapture sret, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #0 {
"function top level":
  %2 = tail call %"enum.std::libc::types::common::c95::c_void[#1]"* @"_ZN2rt11global_heap10malloc_raw17h56c543b77f9b78aY11v0.9$x2dpreE"({ i64, %tydesc*, i8*, i8*, i8 }* undef, i64 8)
  %3 = bitcast %"enum.std::libc::types::common::c95::c_void[#1]"* %2 to i64*
  store i64 5, i64* %3, align 8
  %4 = getelementptr inbounds %"enum.std::option::Option<~int>[#1]"* %0, i64 0, i32 0
  store i64* %3, i64** %4, align 8
  ret void
}

After:

; Function Attrs: uwtable
define %"enum.std::option::Option<~int>[#1]" @_ZN3foo18h2cbf6557a3143edah4v0.0E({ i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #0 {
"function top level":
  %1 = tail call %"enum.std::libc::types::common::c95::c_void[#1]"* @"_ZN2rt11global_heap10malloc_raw18hb1e9dd1beab35edau11v0.9$x2dpreE"({ i64, %tydesc*, i8*, i8*, i8 }* undef, i64 8)
  %2 = bitcast %"enum.std::libc::types::common::c95::c_void[#1]"* %1 to i64*
  store i64 5, i64* %2, align 8
  %oldret = insertvalue %"enum.std::option::Option<~int>[#1]" undef, i64* %2, 0
  ret %"enum.std::option::Option<~int>[#1]" %oldret
}

[thestinger]

I've verified that this works with the threshold set above 1 word, to confirm that this works with normal enum layouts and not just Option<NonNullablePointer>-like ones for when @jld's enum pull request lands.