PR159: Exception handler blocks / register mixup

PR159: Exception handler blocks / register mixup
* src/vm/jit/codegen-common.cpp: Fix register allocation at exception handler
block boundaries.
The concept of a block type is flawed because an exception handler block can
also be reached via normal control flow. I guess subroutine blocks are safe
because they expect a different type on the stack (returnAddress), and any
attempt at reaching them normally would not get past the verifier, although I
don't know how this is handled in CACAO.

The conditional branches should be handled as well, but as there is a lot of
platform-specific code involved as well as lots of register clobbering
potential, I'll leave it at that for now.