[WinEH] Make llvm.x86.seh.recoverfp work on x64

It adjusts from RSP-after-prologue to RBP, which is what SEH filters
need to do before they can use llvm.localrecover.

Fixes SEH filter captures, which were broken in r250088.

Issue reported by Alex Crichton.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@255707 91177308-0d34-0410-b5e6-96231b3b80d8
10 files changed