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[IPSCCP] Use ParamState for arguments at call sites.

Authored by fhahn on Jan 23 2020, 11:17 AM.

Description

[IPSCCP] Use ParamState for arguments at call sites.

We currently use integer ranges to merge concrete function arguments.
We use the ParamState range for those, but we only look up concrete
values in the regular state. For concrete function arguments that are
themselves arguments of the containing function, we can use the param
state directly and improve the precision in some cases.

Besides improving the results in some cases, this is also a small step towards
switching to ValueLatticeElement, by allowing D60582 to be a NFC.

Reviewers: efriedma, davide

Reviewed By: efriedma

Differential Revision: https://reviews.llvm.org/D71836