[IR] Look through bitcast in hasFnAttribute()

A logic incompleteness may lead MemorySSA to be too conservative
in its results. Specifically, when dealing with a call of kind
`call i32 bitcast (i1 (i1)* @test to i32 (i32)*)(i32 %1)`, where
the function `test` is declared with readonly attribute, the
bitcast is not looked through, obscuring function attributes. Hence,
some methods of CallBase (e.g., doesNotReadMemory) could provide
suboptimal results.

Differential Revision: https://reviews.llvm.org/D109888