Teach MachineSinking to handle easy critical edges.

Sometimes it is desirable to sink instructions along a critical edge:

x = ...
if (a && b) ...
else use(x);

The 'a && b' condition creates a critical edge to the else block, but we still
want to sink the computation of x into the block. The else block is dominated by
the parent block, so we are not pushing instructions into new code paths.

llvm-svn: 101165