Fix type/r/r2 confusion differently

Fix type/r/r2 confusion differently

on i386 111_conversion.c breaks when save_reg_upstack isn't careful
about r2 and type mismatches.  The bcheck patches fixed this by
enlarging the stack slot beyond the natural type, this variant simply
avoids saving the second register is the type indicates that it isn't
needed.

Adds also a comment how this should ideally work, namely that type
and r/r2 entries in the vstack are consistent.  In the 111_conversion
testcase it's specifically gen_cast via gen_cvt_ftoi that breaks
this, but there more general code broken as well, so that would deserve
a careful fixup based on some additional asserts.