Load FP immediates by movgr2fr from materialized integer if the bitcasted integer
can be materialized within 2 instructions.
For example, when loading double 1024.0, use
lu52i.d $a0, $zero, 1033 movgr2fr.d $fa0, $a0
instead of
pcalau12i $a0, .LCPI2_0 addi.d $a0, $a0, .LCPI2_0 fld.d $fa0, $a0, 0
Test this patch with 3A5000 on llvm13, the result shows that SPEC CPU2006 FP
score increases 1.2% in average, 470.lbm score increases 11.9%.
Thanks to @xry111 for the suggestion: https://reviews.llvm.org/D128898#3632140
nit: bitcast -- the verb "cast"'s past participle is itself, so is the compound word "bitcast".