llc: Add a 'run-pass' option.

This commit adds a 'run-pass' option to llc, which instructs the compiler to run
one specific code generation pass only.

Llc already has the 'start-after' and the 'stop-after' options, and this new
option complements the other two by making it easier to write tests that want
to invoke a single pass only.

Reviewers: Duncan P. N. Exon Smith

Differential Revision: http://reviews.llvm.org/D10776

llvm-svn: 241476