Unrank mcuMemHostRegister tensor argument.

Reviewers: herhut

Reviewed By: herhut

Subscribers: mehdi_amini, rriddle, jpienaar, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, stephenneuendorffer, Joonsoo, grosul1, frgossen, Kayjukh, jurahul, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D80118