cranelift: Remove redundant branch and select instructions (#5097)

As discussed in the 2022/10/19 meeting, this PR removes many of the branch and select instructions that used iflags, in favor if using brz/brnz and select in their place. Additionally, it reworks selectif_spectre_guard to take an i8 input instead of an iflags input.

For reference, the removed instructions are: br_icmp, brif, brff, trueif, trueff, and selectif.

cranelift/codegen/src/verifier/mod.rs

@@ -639,21 +639,6 @@ impl<'a> Verifier<'a> {
                 destination,
                 ref args,
                 ..
-            }
-            | BranchInt {
-                destination,
-                ref args,
-                ..
-            }
-            | BranchFloat {
-                destination,
-                ref args,
-                ..
-            }
-            | BranchIcmp {
-                destination,
-                ref args,
-                ..
             } => {
                 self.verify_block(inst, destination, errors)?;
                 self.verify_value_list(inst, args, errors)?;
@@ -776,10 +761,7 @@ impl<'a> Verifier<'a> {
             | Shuffle { .. }
             | IntCompare { .. }
             | IntCompareImm { .. }
-            | IntCond { .. }
             | FloatCompare { .. }
-            | FloatCond { .. }
-            | IntSelect { .. }
             | Load { .. }
             | Store { .. }
             | Trap { .. }