[GISel] Add LookThroughInstrs for getIConstantVRegVal and getIConstan… (#68327)

…tVRegSExtVal

The implementation of these methods uses
getIConstantVRegValWithLookThrough with LookThroughInstrs argument set
to false. By adding the optional argument to getIConstantVRegVal and
getIConstantVRegSExtVal we can take advantage of the already built look
through functionality.