[AArch64][GISel] Assign G_LOAD defs into FPR if they feed FP instructions...

[AArch64][GISel] Assign G_LOAD defs into FPR if they feed FP instructions indirectly via PHI (#94618)

If G_LOAD def is used by a PHI, recursively check if the def of PHI is
used by any instruction that only uses FP oprand. If so, assign the def
of G_LOAD to FPR.

In other words, this change helps RBS to assign G_LOAD defs into FPR if
they are later on used in an FP instruction **indirectly via PHI**, to
avoid unnecessary copies. Before this patch, we only considered direct
usages.

It helps to fix GISel regression in TSVC kernel s3110.
GISel was 50% slower than SDAG before this change. Now GISel and SDAG
have the same runtime.