Convert a few assertions with side-effects into regular old runtime checks.

Convert a few assertions with side-effects into regular old runtime checks.
These side-effects seem to make a difference when using llc -march=sparcv9
in Release mode (i.e., with -DNDEBUG); when they are left out, lots of
instructions just get dropped on the floor, because they never end up
in the schedule.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@15339 91177308-0d34-0410-b5e6-96231b3b80d8