[PATCH]: combine.c SHIFT_COUNT_TRUNCATED adjustment

[Clarke, Stephen]

The attached patch to combine.c makes shift count truncation
more effective for targets where the shift count has a different
mode than the expression being shifted.

Originally described at 

http://gcc.gnu.org/ml/gcc/2002-06/msg00379.html

where (for sh64) the shift count truncation code was ineffective
on:

(insn/i 41 40 42 (set (reg:DI 177)
        (zero_extend:DI (reg/v:SI 159))) 114 {zero_extendsidi2} (insn_list 6 (nil))
    (expr_list:REG_DEAD (reg/v:SI 159)
        (nil)))

(insn/i 42 41 43 (set (reg:V2SI 178)
        (ashift:V2SI (reg:V2SI 176)
            (reg:DI 177))) 358 {ashlv2si3} (insn_list 40 (insn_list 41 (nil)))
    (expr_list:REG_DEAD (reg:V2SI 176)
        (expr_list:REG_DEAD (reg:DI 177)
            (nil))))

because it was trying to force the shift count to V2SI
mode.  That didn't match any instruction patterns.

This is applicable to any target which has
SHIFT_COUNT_TRUNCATED defined true, and shift patterns in
which the shift count is a different mode than the expression
being shifted.

Tested on sh64 and mips64, where it did not introduce any new
regressions (words chosen carefully, as both those targets have
a number of failures at the moment).

Steve.

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