[NewPM] Rename 'unswitch' to 'simple-loop-unswitch' in PassRegistry

It is confusing to have two ways of specifying the same pass
('simple-loop-unswitch' and 'unswitch'). This patch replaces
'unswitch' by 'simple-loop-unswitch' to get a unique identifier.

Using 'simple-loop-unswitch' instead of 'unswitch' also has the
advantage of matching how the pass is named in DEBUG_TYPE etc. So
this makes it a bit more consistent how we refer to the pass in
options such as -passes, -print-after and -debug-only.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D105628