[WebAssembly] Use dominator information to improve BLOCK placement

[WebAssembly] Use dominator information to improve BLOCK placement

Always starting blocks at the top of their containing loops works, but creates
unnecessarily deep nesting because it makes all blocks in a loop overlap.
Refine the BLOCK placement algorithm to start blocks at nearest common
dominating points instead, which significantly shrinks them and reduces
overlapping.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@253876 91177308-0d34-0410-b5e6-96231b3b80d8