Fix helptext for opt/llc after 14fc20ca

The commit https://reviews.llvm.org/rG14fc20ca6 added some options to the X86
back end that cause the help text for opt/llc to become much harder to read.
The issue is that the cl::value_desc is part of the option name and is used to
compute the indentation of the description text (i.e. the maximum length option
name is what everything aligns to). Since the commit puts a large number of
characters into that text, everything is aligned to that width.

This patch just reformats the option so that the description is contained in the
description and the list of possible values is within the angle brackets.

Note: the readability issue of the helptext was fixed in commit
      70cbf8c7, but the re-formatting wasn't
      added on that commit so I am still committing this.

Differential revision: https://reviews.llvm.org/D73267