[FPEnv] Add an option to disable strict float node mutating to an normal

float node

This patch add an option 'disable-strictnode-mutation' to prevent strict
node mutating to an normal node.
So we can make sure that the patch which sets strict-node as legal works
correctly.

Patch by Chen Liu(LiuChen3)

Differential Revision: https://reviews.llvm.org/D70226