[InstCombine] Replace an 'if' that should always be true with an assert.

The bitwidth of the operation should always be wider than the result width of the truncate since we don't recurse through any width changing operations.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@332055 91177308-0d34-0410-b5e6-96231b3b80d8