Always use -Wl,-gc-sections on our build.

Both bfd ld and gold correctly handle --export-dynamic, so gc-sections is safe even for binaries
that support plugins.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220174 91177308-0d34-0410-b5e6-96231b3b80d8
Add a datalayout string to this test so that it exercises the full gamut
of InstCombine rather than just the bits enabled when datalayout is
optional.

The primary fixes here are because now things are little endian.

In good news, silliness like this seems like it will be going away as
we've got pretty stong consensus on dropping optional datalayout
entirely.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220176 91177308-0d34-0410-b5e6-96231b3b80d8
Teach the load analysis driving core instcombine logic and other bits of
logic to look through pointer casts, making them trivially stronger in
the face of loads and stores with intervening pointer casts.

I've included a few test cases that demonstrate the kind of folding
instcombine can do without pointer casts and then variations which
obfuscate the logic through bitcasts. Without this patch, the variations
all fail to optimize fully.

This is more important now than it has been in the past as I've started
moving the load canonicialization to more closely follow the value type
requirements rather than the pointer type requirements and thus this
needs to be prepared for more pointer casts. When I made the same change
to stores several test cases regressed without logic along these lines
so I wanted to systematically improve matters first.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@220178 91177308-0d34-0410-b5e6-96231b3b80d8