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

[thestinger]

fn foo() -> (u32, u8, u8, u8, u8) {
    (4, 5, 6, 7, 8)
}

Before:

; Function Attrs: nounwind uwtable
define void @_ZN3foo18hbb616262f874f8daf4v0.0E({ i32, i8, i8, i8, i8 }* noalias nocapture sret, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #0 {
"function top level":
  %2 = getelementptr inbounds { i32, i8, i8, i8, i8 }* %0, i64 0, i32 0
  store i32 4, i32* %2, align 4
  %3 = getelementptr inbounds { i32, i8, i8, i8, i8 }* %0, i64 0, i32 1
  store i8 5, i8* %3, align 4
  %4 = getelementptr inbounds { i32, i8, i8, i8, i8 }* %0, i64 0, i32 2
  store i8 6, i8* %4, align 1
  %5 = getelementptr inbounds { i32, i8, i8, i8, i8 }* %0, i64 0, i32 3
  store i8 7, i8* %5, align 2
  %6 = getelementptr inbounds { i32, i8, i8, i8, i8 }* %0, i64 0, i32 4
  store i8 8, i8* %6, align 1
  ret void
}

After:

; Function Attrs: nounwind readnone uwtable
define { i32, i8, i8, i8, i8 } @_ZN3foo18hbb616262f874f8daf4v0.0E({ i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone) #0 {
"function top level":
  ret { i32, i8, i8, i8, i8 } { i32 4, i8 5, i8 6, i8 7, i8 8 }
}

[thestinger]

I am still having a bit of trouble tracking down problematic struct and enum code paths, so we'll start with this.