Avoid redundant loads in capture generation

Avoid redundant loads in capture generation

When generating the SSA form for captures, the previous code generated
loads of all the variables that existed. This worked correctly for
some cases, but since it skipped the system in trans_expr that forwards
loads, it would sometimes generate loads to SSA variables that could
just be directly referenced instead, which would end up causing loads
from stack slots that were never written to due to optimizations.

Using trans_expr on the captured variable instead of generating loads
directly lets us reuse that capture logic, and it ends up making the
code simpler :D