Entry blocks have an extra predecessor, part 2

Summary: Similar to D1412834, we weren't counting the extra predecessor that
entry blocks have in JIT::Block.

Reviewed By: @ottoni

Differential Revision: D1425851
Fix asserts with Mov codegen.

Summary: 1. XLS won't allocate registers for InitNull, but it isn't treated as
a const, so we'd die when trying to do Type::rawVal() in cgMov().

2. t2:StkPtr = Mov t1:StkPtr is legal, but we weren't handling Mov in
forceAlloc().

3. cgMov wasn't handling the dest register being InvalidReg.

Reviewed By: @ottoni

Differential Revision: D1425846