[XRay] Fix assertion failure on empty machine basic blocks (PR 31424)

The original version of the code in XRayInstrumentation.cpp assumed that
functions may not have empty machine basic blocks (or that the first one
couldn't be). This change addresses that by special-casing that specific
situation.

We provide two .mir test-cases to make sure we're handling this
appropriately.

Fixes llvm.org/PR31424.

Reviewers: chandlerc

Subscribers: varno, llvm-commits

Differential Revision: https://reviews.llvm.org/D27913

llvm-svn: 290091