Avoid a redundant MOVE on some string interpolation setups

[9382]

This fixes a discrepancy between string interpolation (local a = `{xyz}`) and the manually written equivilant version (local a = ("%*"):format(xyz)), where string interpolation would sometimes introduce a MOVE even when it didn't need to as the target register was marked as temporary

Bytecode difference

Code: local f = `{global}`

Before:

Function 0 (??):
    1: local f = `{global}`
LOADK R1 K0 ['%*']
GETGLOBAL R3 K1 ['global']
NAMECALL R1 R1 K2 ['format']
CALL R1 2 1
MOVE R0 R1
RETURN R0 0

After:

Function 0 (??):
    1: local f = `{global}`
LOADK R0 K0 ['%*']
GETGLOBAL R2 K1 ['global']
NAMECALL R0 R0 K2 ['format']
CALL R0 2 1
RETURN R0 0

This is also technically a performance improvement, though the difference is mostly unnoticeable in normal contexts (I'm seeing a 4-5% improvement on a 1e7 loop which is nothing but the optimisable string interp, so about as good as the case gets)

I've not written a new test case as InterpStringRegisterCleanup already covers the unoptimisable case

[tommyscholly]

This looks good, great catch on the redundant move. Just need the changes to be flagged.

[9382]

(Merged with master to resolve a conflict in Compiler, hopefully it isn't too much of a mess git-wise)

[SPY]

LGTM