[WebAssembly] Remove getBottom function from CFGStackify (NFC)

Summary:
This removes `getBottom` function and the bookeeping map of <begin
marker instruction, bottom BB>.

Reviewers: dschuff

Subscribers: sunfish, sbc100, jgravelle-google, jdoerfert, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D58319

llvm-svn: 354657