[Hexagon] Remove unintended fallthrough from MC duplex code

I added these annotations in r345878 because I wasn't sure if the
fallthrough was intended. Krzysztof Parzyszek confirmed that they should
be breaks, so that's what this patch does.

Reviewers: kparzysz

Differential Revision: https://reviews.llvm.org/D53991

llvm-svn: 345883