Systematic treatment of (potentially too large) immediates

[eponier]

Currently, during lowering, we transform some instructions (in particular additions) if the immediate they take as an argument is too large to be taken directly. We then introduce a move of this immediate to a register, and replace the immediate by this register in the original instruction. This approach has several drawbacks:

• this introduces an extra instruction without the user noticing -> the compiler should at least emit a warning
• this is done only for some instructions in an ad-hoc manner -> it should be done in a systematic way, dealing both with the cases where immediates are not allowed and the cases where immediates are allowed with an upper bound

[eponier]

This could be done in a different pass after lowering.

[eponier]

Actually, it has been decided that the pass must be optional, and disabled when we use the "strict mode".