rs6000: Define movsf_from_si2 to extract high part SF element from DImode[PR89310]

rs6000: Define movsf_from_si2 to extract high part SF element from DImode[PR89310]

For extracting high part element from DImode register like:

{%1:SF=unspec[r122:DI>>0x20#0] 86;clobber scratch;}

split it before reload with "and mask" to avoid generating shift right
32 bit then shift left 32 bit.  This pattern also exists in PR42475 and
PR67741, etc.

srdi 3,3,32
sldi 9,3,32
mtvsrd 1,9
xscvspdpn 1,1

=>

rldicr 3,3,0,31
mtvsrd 1,3
xscvspdpn 1,1

Bootstrap and regression tested pass on Power8-LE.

gcc/ChangeLog:

2020-07-21  Xionghu Luo  <luoxhu@linux.ibm.com>

	PR rtl-optimization/89310
	* config/rs6000/rs6000.md (movsf_from_si2): New define_insn_and_split.

gcc/testsuite/ChangeLog:

2020-07-21  Xionghu Luo  <luoxhu@linux.ibm.com>

	PR rtl-optimization/89310
	* gcc.target/powerpc/pr89310.c: New test.