Changes to enhance Chisel 3 compatibility

These were all SInt/UInt constants or cast changes.  I believe these changes
should do nothing in Chisel 2 -- they're all just things that have been defined
as illegal in Chisel 3 because they were implementation defined in Chisel 2.  I
changed these to explicitly match the Chisel 2 implementation's behavior.

"make test-c" still passes.