[LVI] A better fix for the assertion error introduced by 267609

Essentially, I was using the wrong size function.  For types which were sized, but not primitive, I wasn't getting a useful size for the operand and failed an assert.  I fixed this, and also added a guard that the input is a sized type.  Test case is for the original mistake.  I'm not sure how to actually exercise the sized type check.

llvm-svn: 267618