[DAGCombiner] fold select-of-constants to shift

This reverses the scalar canonicalization proposed in D63382.

Pre: isPowerOf2(C1)
%r = select i1 %cond, i32 C1, i32 0
=>
%z = zext i1 %cond to i32
%r = shl i32 %z, log2(C1)

https://rise4fun.com/Alive/Z50

x86 already tries to fold this pattern, but it isn't done
uniformly, so we still see a diff. AArch64 probably should
enable the TLI hook to benefit too, but that's a follow-on.

llvm-svn: 374397