[test][InstCombine] Add tests for removing memcpy to an alloca that is passed...

[test][InstCombine] Add tests for removing memcpy to an alloca that is passed to a readonly nocapture function parameter, in preparation for D136822.

This commit adds tests to Transforms/InstCombine/memcpy-from-global.ll that
test various situations involving memcpy from a constant to an alloca that is
then passed to function parameters with various attributes. The forthcoming
D136822 allows InstCombine to remove these memcpys if they're passed to a
single readonly nocapture parameter.

Differential Revision: https://reviews.llvm.org/D137033