Split memcpy/memset/memmove intrinsics into i32/i64 versions, resolving

PR709, and paving the way for future progress.

Significantly refactor autoupgrading code, to handle the more complex case
(where we upgrade one argument in a function), and fix some bugs in it.

Testcase here: llvm/test/Regression/Bytecode/memcpy.ll

llvm-svn: 26474