[GVN] Support load of pointer-select to value-select conversion.

This patch extends the available-value logic to detect loads
of pointer-selects that can be replaced by a value select.

For example, consider the code below:

  loop:
    %sel.phi = phi i32* [ %start, %ph ], [ %sel, %ph ]
    %l = load %ptr
    %l.sel = load %sel.phi
    %sel = select cond, %ptr, %sel.phi
    ...

  exit:
    %res = load %sel
    use(%res)

The load of the pointer phi can be replaced by a load of the start value
outside the loop and a new phi/select chain based on the loaded values,
as illustrated below

    %l.start = load %start
  loop:
    sel.phi.prom = phi i32 [ %l.start, %ph ], [ %sel.prom, %ph ]
    %l = load %ptr
    %sel.prom = select cond, %l, %sel.phi.prom
    ...
  exit:
    use(%sel.prom)

This is a first step towards alllowing vectorizing loops using common libc++
library functions, like std::min_element (https://clang.godbolt.org/z/6czGzzqbs)

    #include <vector>
    #include <algorithm>

    int foo(const std::vector<int> &V) {
        return *std::min_element(V.begin(), V.end());
    }

Reviewed By: reames

Differential Revision: https://reviews.llvm.org/D118143