CodeGenPrep: sink extends of illegal types into use block.

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

This is an update of D2973 which was reverted because of a bug reported
as PR19084.

Reviewers: t.p.northover, chapuni

Reviewed By: t.p.northover

CC: llvm-commits, alex, chapuni

Differential Revision: http://llvm-reviews.chandlerc.com/D3021