Improve additions involving SP on Thumb-1

[Bernd Schmidt <bernds at codesourcery dot com>]

Thumb-1 supports a range of up to 1020 for add insns of the form

  add rn, sp, #1020

If the constant is somewhat larger, we can do it with a sequence of two
adds, which is better than the available alternatives:

-       ldr     r4, .L64+12
-       add     r4, r4, sp
+       add     r4, sp, #1020
+       add     r4, r4, #224

or

-       mov     r0, #156
-       lsl     r0, r0, #3
-       add     r0, r0, sp
+       add     r0, sp, #1020
+       add     r0, r0, #228

It's conceivable that we might even prefer a three-insn add sequence
over either of these alternatives, but the following patch doesn't do this.

Regression tested on arm-linux, with one extra timeout (which comes and
goes - it went away vs. baseline for a different set of compiler flags).
 Ok?


Bernd

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