[DAGCombiner] Stop visitEXTRACT_SUBVECTOR creating illegal BITCASTs post legalisation.

visitEXTRACT_SUBVECTOR can sometimes create illegal BITCASTs when
removing "redundant" INSERT_SUBVECTOR operations.  This patch adds
an extra check to ensure such combines only occur after operation
legalisation if any resulting BITBAST is itself legal.

Differential Revision: https://reviews.llvm.org/D108086

(cherry picked from commit cd0e1964)