Reapply "[RISCV] Enable strict assertions in InsertVSETVLI data flow"

be2cb8 fixes the case which triggered the revert.  Reapply, and let's see if anything else falls out.

Original commit message:

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

llvm/lib/Target/RISCV/RISCVInsertVSETVLI.cpp

@@ -38,7 +38,7 @@ static cl::opt<bool> DisableInsertVSETVLPHIOpt(
     cl::desc("Disable looking through phis when inserting vsetvlis."));
 
 static cl::opt<bool> UseStrictAsserts(
-    "riscv-insert-vsetvl-strict-asserts", cl::init(false), cl::Hidden,
+    "riscv-insert-vsetvl-strict-asserts", cl::init(true), cl::Hidden,
     cl::desc("Enable strict assertion checking for the dataflow algorithm"));
 
 namespace {