We have trouble with Math.fround(MEM[...]).  Since we now properly type
LoadBuffer (it can produce undefined), lowering of fround has stopped
triggering (as it requires Number type).  This CL is a quick fix for this issue
- we simply trigger the lowering for NumberOrUndefined and let representation
selection/truncation analysis deal with this.

Ultimately, we would want to insert some 'simplified' ToNumber conversion
that would be optimized as much as possible during representation
selection.

BUG=chromium:603802
LOG=n
R=bmeurer@chromium.org

Review URL: https://codereview.chromium.org/1893483003

Cr-Commit-Position: refs/heads/master@{#35513}