[WinEH] Mark funclet entries and exits as clobbering all registers

Summary:
In this implementation, LiveIntervalAnalysis invents a few register
masks on basic block boundaries that preserve no registers. The nice
thing about this is that it prevents the prologue inserter from thinking
it needs to spill all XMM CSRs, because it doesn't see any explicit
physreg defs in the MI.

Reviewers: MatzeB, qcolombet, JosephTremoulet, majnemer

Subscribers: MatzeB, llvm-commits

Differential Revision: http://reviews.llvm.org/D14407

llvm-svn: 252318