[patch, CSE] Bit-field insertion optimization

[Andrew Stubbs]

The attached patch fixes a bug in which constant assignments to bit 
fields are improperly optimized. I'm seeing this on ARM, but I imagine 
it affects other targets similarly.

The first problem is that CSE cannot determine that the result is 
constant because the auto-variable is implicitly initialized. I have 
solved this by moving up the init-regs pass to before cse2. It might be 
better to move it before cse1, but that's a bigger change, so I wasn't sure?

The second problem is that the pattern match for ZERO_EXTRACT requires 
that the operand is an immediate constant, which is never the case on 
ARM (and presumably is only the case with a limited range of inputs even 
on other targets). I have added code to detect known-constant input 
registers.

Test case:

   struct bits
   {
      unsigned a:5;
      unsigned b:5;
      unsigned c:5;
      unsigned d:5;
   };

   struct bits
   f (unsigned int a)
   {
      struct bits bits = {0,0,0,0};
      bits.a = 1;
      bits.b = 2;
      bits.c = 3;
      bits.d = a;
      return bits;
   }


Before, compiled for ARM with "-O2 -mcpu=cortex-a8 -mthumb":
         movs    r2, #1
         movs    r3, #0
         bfi     r3, r2, #0, #5
         movs    r2, #2
         bfi     r3, r2, #5, #5
         movs    r2, #3
         bfi     r3, r2, #10, #5
         bfi     r3, r0, #15, #5
         mov     r0, r3
         bx      lr

After:
         movw    r3, #3137
         bfi     r3, r0, #15, #5
         mov     r0, r3
         bx      lr

OK for commit, once stage 1 opens again?

Andrew

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