[Stackmap] Allow WebKit_JS calling convention to store 4 byte sized and aligned arguments.

This allows the WebKit_JS calling convention to perform partial writes on a 4
byte granularity to stack slots.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@197431 91177308-0d34-0410-b5e6-96231b3b80d8