Linker performance problems

Linking a hello world with:
monolinker -a hello.exe -c link
Takes about 4.7 seconds with 8a82325ce8f34ed02655b834f4c6821b9d8f7011. This is a problem since the linker is on a critical path of the build and its ran serially. This used to be about 1-2 seconds. Link time should be proportional to the size of the final app.

Notes:
* Adding --disable-opt ipconstprop reduces the run time to 3.3 secs.
* The TypeMapStep loads and processes the whole unlinked assembly.
* Multiple passes are made over the IL, i.e. in MethodBodyScanner.