[IRCE] Avoid computing potentially unnecessary analyses. NFC

IRCE is a function pass that operates on loops. If there are no loops in
the function (as seen through LI), we should avoid computing the
remaining expensive analyses (such as BPI). Reordered the analyses
requests and early return if there are no loops. This is an NFC with
compile time improvement.

The same will be done in a follow-up patch for the loop vectorizer.

Reviewed-By: nikic
Differential Revision: https://reviews.llvm.org/D124478