[SelectionDAGBuilder] Compute and cache PreferredExtendType on demand.

Previously we pre-calculated this and cached it for every
instruction in the function. Most of the calculated results will
never be used. So instead calculate it only on the first use, and
then cache it.

The cache was originally added to fix a compile time issue which
caused r216066 to be reverted.

This change exposed that we weren't pre-computing the Value for
Arguments. I've explicitly disabled that for now as it seemed to
regress some tests on AArch64 which has sext built into its compare
instructions.

Spotted while investigating how to improve heuristics to work better
with RISCV preferring sign extend for unsigned compares for i32 on RV64.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D107976