Two fixes for compact unwind decoding for frameless large-stack-size

i386/x86_64 functions.  The stack size was being multiplied by the
pointer size incorrectly.  The register permutation placeholders
(UNWIND_X86_REG_NONE) were decrementing the stack offset of the
saved registers when it should not have been.

<rdar://problem/19570035> 

llvm-svn: 226889