Introduce frame.llvmvalue and port all clients which do not use govalue to it
Modify frame.slice to take llvm.Values instead of govalues
Avoid loading large structures when we are only going to use them to create an interface

This may look like an optimization, but it helps us avoid assertion failures
in the LLVM backend resulting from loads of large structures.
Modify interfaceMethod to take llvm.Values instead of govalues

Also, use the supplied interface type instead of the receiver of the
interface method. This avoids problems with embedded interfaces, and fixes
11 libgo tests.
Merge pull request #163 from pcc/libgo-llvmvalue

Introduce frame.llvmvalue and start porting clients to it