JIT: Simplify redundant if statements for memcheck.
JIT: unify two branches in SafeLoadToReg.

Should behave the same as it did before.
Remove unnecessary memcheck check.

If we make the memory read call return the correct value, we don't need
the extra test+branch.
Remove another unnecessary memcheck check.
Make fastmem loads work with memcheck.

Preserve the value in the register if the load causes an exception.