Fix a number of byval / memcpy / memset related codegen issues.

Fix a number of byval / memcpy / memset related codegen issues.
1. x86-64 byval alignment should be max of 8 and alignment of type. Previously the code was not doing what the commit message was saying.
2. Do not use byte repeat move and store operations. These are slow.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@55139 91177308-0d34-0410-b5e6-96231b3b80d8