Fix codegen bug in PIC version of genSwitch (#12433)

Summary:
* getNonClobberedReg instead of getSomeReg, because the reg needs to
  survive across t_code
* Use a new reg for the table offset calculation instead of clobbering
  the reg returned by expr (this was the bug affecting #12433)

Test Plan: New unit test; validate

Reviewers: rwbarton, bgamari, austin, erikd

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D2529

GHC Trac Issues: #12433