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/isa/x64/lower/isle.rs

@@ -593,16 +593,6 @@ impl Context for IsleContext<'_, '_, MInst, Flags, IsaFlags, 6> {
         }
     }
 
-    #[inline]
-    fn intcc_reverse(&mut self, cc: &IntCC) -> IntCC {
-        cc.reverse()
-    }
-
-    #[inline]
-    fn floatcc_inverse(&mut self, cc: &FloatCC) -> FloatCC {
-        cc.inverse()
-    }
-
     #[inline]
     fn sum_extend_fits_in_32_bits(
         &mut self,