Revert "X86: Align the stack on word boundaries in LowerFormalArguments()"

This (mostly) reverts commit r216119.

Somewhere during the review Reid committed r214980 which fixed this
another way, and I neglected to check that the testcase still failed
before committing.

I've left test/CodeGen/X86/aligned-variadic.ll around in case it adds
extra coverage.

llvm-svn: 216246