[CIR] Generate SelectOp instead of TernaryOp for if cheap enough to evaluate unconditionally

[mmha]

This came up during the review of llvm/llvm-project#138156

During codegen we check whether the LHS and RHS of the conditional operator are cheap enough to evaluate uncondionally. Unlike classic codegen we still emit TernaryOp instead of SelectOp and defer that optimization to cir-simplify.

This patch changes codegen to directly emit SelectOp for cond ? constant : constant expressions.

[mmha]

I deleted clang/test/CIR/Transforms/ternary-fold.cpp because all test cases inside that file are now handled by codegen so cir-simlify has nothing to do here. I do wonder whether SimplifyTernary still has a use.

[bcardosolopes]

Thanks Morris! LGTM