Fix IRA issue, PR79728

[Bernd Schmidt]

This is an ICE where setup_pressure_classes fails if xmm0 is a global 
reg. Instead of GENERAL/FLOAT/SSE/MMX_REGS, it computes only 
SSE_FIRST_REG as the third register class. The problem is that the costs 
for moving between SSE_FIRST_REG and SSE_REGS are inflated because we 
think we have no available registers in SSE_FIRST_REG (since the only 
register in that class is global), and that somewhat confuses the algorithm.

The following fixes it by tweaking contains_regs_of_mode. Out of 
caution, I've retained the old meaning for code in reload which uses this.

Bootstrapped and tested on x86_64-linux. Ok?


Bernd
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