[SCCP] Support ranges for loads and stores.

Integer ranges can be used for loaded/stored values. Note that widening
can be disabled for loads/stores, as we only rely on instructions that
cause continued increases to ranges to be widened (like binary
operators).

Reviewers: efriedma, mssimpso, davide

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D78433