[GISel] Make the PartialMapping and ValueMapping constructors constexpr.

Backends contain static tables of PartialMapping and ValueMapping.
Without the constexpr, the initial code from the frontend contains
a global constructor to construct each entry of the table. The clang
optimizer is able to optimize the constructors away, but gcc 8 is not
able to.

Using constexpr allows the tables to be created directly in the
frontend without requiring optimizations to be performed.