[X86] Fix arg copy elision for illegal types

[X86] Fix arg copy elision for illegal types

Use the store size of the argument type, which will be a byte-sized
quantity, rather than dividing the size in bits by 8.

Fixes PR32136 and re-enables copy elision from i64 arguments.

Reverts the workaround in from r296950.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@297045 91177308-0d34-0410-b5e6-96231b3b80d8