[SDAG/ISel] clmul lowering for higher bitwidth produces suboptimal result

[artagnon]

Actual result: https://godbolt.org/z/577Wb9E99
Desired result: https://godbolt.org/z/hbEe3WMdW

Ref: rust-lang/rust#157831 (thanks to @folkertdev)

CC: @RKSimon, @topperc, @eisenwave, @xarkenz.