Using sha256 to compare strings is stressing out the register allocator.

[otrho]

In #4336 I had to massage both the stdlib/vec and test_contracts/basic_storage tests to isolate parts which were comparing strings. They were using assert(sha256(str_a) == sha256(str_b)). For some reason putting a few of these together would use up all the registers.

I have a hunch it's because both sha256 and assert use ASM blocks which are optimisation blockers. But there might be a simple solution.

The register allocator is to be rewritten next so this isn't necessarily high priority, but if there is a simple fix then it's worth taking a look.