MSan: handle callbr instructions

Summary:
Handling callbr is very similar to handling an inline assembly call:
MSan must checks the instruction's inputs.
callbr doesn't (yet) have outputs, so there's nothing to unpoison,
and conservative assembly handling doesn't apply either.

Fixes PR42479.

Reviewers: eugenis

Subscribers: hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64072

llvm-svn: 365008