Clean input group flex basis

Created by: ysds

Fixes #28782 (closed)

• The width: 1% overwrites the default width: 100% of .form-control etc.
• But the width: 1% can be replaced with flex-basis, because flex-basis has priority (ref: https://developer.mozilla.org/en-US/docs/Web/CSS/flex-basis)
• % is needed for IE bug https://github.com/philipwalton/flexbugs#flexbug-4

I've tested on https://codepen.io/fellows3/pen/Ezpdqj