[LV] Changing the interface of ValueMap, NFC.

[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

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306331 91177308-0d34-0410-b5e6-96231b3b80d8