Add support to paternmatch for simple const Value cases.

Pattern match has some paths which can operate on constant instructions,
but not all.  This adds a version of m_value() to return const Value* and
changes ICmp matching to use auto so that it can match both constant and
mutable instructions.

Tests also included for both mutable and constant ICmpInst matching.

This will be used in a future commit to constify ValueTracking.cpp.

llvm-svn: 278570