Change weight into a float so that we can take into account the

Change weight into a float so that we can take into account the
nesting level when computing it. Right now the allocator uses:

    w = sum_over_defs_uses( 10 ^ nesting level );

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@10569 91177308-0d34-0410-b5e6-96231b3b80d8