cranelift: charge Else and End operator fuel in target blocks

[ctiller]

FuncEnvironment::fuel_before_op previously added the operator cost for every instruction other than Nop and Drop before translate_operator ran, guarded by state.reachable at instruction entry. For Operator::Else and Operator::End, this caused two bugs when custom OperatorCost values were configured:

1. At entry to Operator::Else, builder is still positioned at the end of the then block before translate_operator emits the jump to exit_block and switches builder to the else block. Charging Else in fuel_before_op therefore charged Else when the then branch was taken and failed to charge Else when the else branch was taken.
2. When the then block (or the body preceding End) ended with an unconditional control transfer (return, br, etc.), state.reachable was false entering Else or End, so before_translate_operator skipped fuel_before_op even when the else block or merged End block was reachable via another incoming edge.

Defer Operator::Else and Operator::End fuel charging from fuel_before_op to fuel_after_op, after translate_operator has switched builder to the target block and updated state.reachable.

Assisted-by: Gemini