Store some variables in registers instead of always using the stack

[NotAFlyingGoose]

Problem

Currently, all local variables are stored in stack slots. This can create unnecessary stack stores and loads even when a variable's address is never obtained

Proposed Solution

codegen should check to see if a variable is ever aliased with ^. This could probably be done in either the hir or hir_ty crates during lowering or type checking. Maybe when an Expr::Ref is created/parsed it would check to see if it's inner expression was a Variable before marking that variable as "aliased".

codegen would then check this flag (and Ty::is_aggregate) to know if it should allocate stack space for the variable or if it should instead use cranelift_frontend::Variable

Notes

Future optimizations could probably be made if it's known that a variable is never aliased. Structs could be split up into separate cranelift_frontend::Variables for their different fields. Although this would also need to check if a variable was passed as an argument.

[lenawanel]

In the same stroke it would probably be good to perform stack_addr instructions only when absolutely necessary.
probably would lead to cranelift having to use less fuel on analyzing them.
This would probably be possible by having functions like compile_expr_with_args return an enum containing either a stack slot or a value.

[NotAFlyingGoose]

I'm gonna start working on this