Fix several issues with alignment. We weren't always accounting for type

Fix several issues with alignment. We weren't always accounting for type
alignment requirements of the new alloca. As one consequence which was
reported as a bug by Duncan, we overaligned memcpy calls to ranges of
allocas after they were rewritten to types with lower alignment
requirements. Other consquences are possible, but I don't have any test
cases for them.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@164937 91177308-0d34-0410-b5e6-96231b3b80d8