[AVR][NFC] Simplify branch relaxation tests

[Patryk27]

Let's use the .space directive to simplify AVR's branch relaxation tests, as noticed by @MaskRay at #118015 (comment).

Having .space 4100 instead of .space 4096 is little bit awkward, but I'd rather avoid changing two things at once (i.e. current assertions are fine).

[benshi001]

LGTM! But it would be better to change the title to [AVR][NFC] Simplify branch relaxation tests.

[Patryk27]

Sure, sounds good!

[MaskRay]

The two tests can be merged into one branch-relaxation.ll, similar to other targets.

[Patryk27]

Yes~no - they are separate, because they check this:

llvm-project/llvm/lib/Target/AVR/AVRDevices.td

Line 63 in 1f96788

// The device potentially requires emitting rjmp that wraps across the flash

(aka #118015)

Merging them into a single file would probably work, but it'd be tiiiny bit illegal, because the generated binary would technically be larger than 8 kB that the lowest target provides (I could imagine LLVM refusing to compile such code in the future, for instance).