If we're falling back to generic constant formation in a register +
add/sub, we can check if we have a constant which is 12-bits but left
shifted by 2 or 3. If so, we can use a sh2add or sh3add to perform the
shift and add in a single instruction.
This is profitable when the unshifted constant would require two
instructions (LUI/ADDI) to form, but is never harmful since we're going
to need at least two instructions regardless of the constant value.
Since stacks are aligned to 16 bytes by default, sh3add allows addresing
(aligned) data out to 2^14 (i.e. 16kb) in at most two instructions
w/zba.
Adds two sets of tests. First, one for prolog/epilogue insertions where
the second stack adjustment can be done with shNadd for zba. Second, a
set of tests with offsets off SP in the same ranges, but also adding
varying alignments.
