MC: Support aligned COMMON symbols for COFF

link.exe:
Fuzz testing has shown that COMMON symbols with size > 32 will always
have an alignment of at least 32 and all symbols with size < 32 will
have an alignment of at least the largest power of 2 less than the size
of the symbol.

binutils:
The BFD linker essentially work like the link.exe behavior but with
alignment 4 instead of 32.  The BFD linker also supports an extension to
COFF which adds an -aligncomm argument to the .drectve section which
permits specifying a precise alignment for a variable but MC currently
doesn't support editing .drectve in this way.

With all of this in mind, we decide to play a little trick: we can
ensure that the alignment will be respected by bumping the size of the
global to it's alignment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218201 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Teach the new vector shuffle lowering the first step toward more
actual support for complex AVX shuffling tricks. We can do independent
blends of the low and high 128-bit lanes of an avx vector, so shuffle
the inputs into place and then do the blend at 256 bits. This will in
many cases remove one blend instruction.

The next step is to permute the low and high halves in-place rather than
extracting them and re-inserting them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218202 91177308-0d34-0410-b5e6-96231b3b80d8
[x86] Fix a helper to reflect that what we actually care about is
128-bit lane crossings, not 'half' crossings. This came up in code
review ages ago, but I hadn't really addresesd it. Also added some
documentation for the helper.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@218203 91177308-0d34-0410-b5e6-96231b3b80d8