Replace as casts with type-conversion functions.

https://github.com/rust-lang-nursery/rust-clippy/wiki#cast_lossless

lib/cretonne/src/isa/intel/abi.rs

@@ -224,7 +224,7 @@ pub fn native_prologue_epilogue(func: &mut ir::Function, isa: &TargetIsa) -> res
     });
 
     let total_stack_size = layout_stack(&mut func.stack_slots, stack_align)? as i32;
-    let local_stack_size = (total_stack_size - csr_stack_size) as i64;
+    let local_stack_size = i64::from(total_stack_size - csr_stack_size);
 
     // Add CSRs to function signature
     let fp_arg = ir::AbiParam::special_reg(

lib/cretonne/src/isa/riscv/binemit.rs

@@ -106,7 +106,7 @@ fn put_i<CS: CodeSink + ?Sized>(bits: u16, rs1: RegUnit, imm: i64, rd: RegUnit,
 ///
 /// Encoding bits: `opcode[6:2] | (funct3 << 5)`
 fn put_u<CS: CodeSink + ?Sized>(bits: u16, imm: i64, rd: RegUnit, sink: &mut CS) {
-    let bits = bits as u32;
+    let bits = u32::from(bits);
     let opcode5 = bits & 0x1f;
     let rd = u32::from(rd) & 0x1f;