CodeGenPrep: sink extends of illegal types into use block.

This helps the instruction selector to lower an i64 * i64 -> i128
multiplication into a single instruction on targets which support it.

Patch by Manuel Jacob.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203230 91177308-0d34-0410-b5e6-96231b3b80d8