Add a knob to WhileLoopConstantSinking to only sink scalar constants.
Add a knob to WhileLoopConstantSinking to only sink scalar constants.
We use this to prepare the module for urolling while loops.

FUTURE_COPYBARA_INTEGRATE_REVIEW=https://github.com/openxla/xla/pull/10631 from Intel-tensorflow:yimei/fix_cpu_bf16_support 9b4f9136f1976b4432e1a60f2deb4a17c150dd3f
