[RISCV] Enable strict assertions in InsertVSETVLI data flow

These asserts are believed to hold after several recent miscompiles have been fixed.  If you see an assertion failure on this change, please toggle the default back and make sure you file a bug with a reproducer.  We may have as yet uncaught miscompiles lurking in this code.

Differential Revision: https://reviews.llvm.org/D125271