[LV] Changing the interface of ValueMap, NFC.

Instead of providing access to the internal MapStorage holding all Values
associated with a given Key, used for setting or resetting them all together,
ValueMap keeps its MapStorage internal; its new interface allows getting,
setting or resetting a single Value, per part or per part-and-lane.
Follows the discussion in https://reviews.llvm.org/D32871.

Differential Revision: https://reviews.llvm.org/D34473

llvm-svn: 306331