Allow matching extend-from-memory with strict FP nodes

This implements a small enhancement to https://reviews.llvm.org/D55506

Specifically, while we were able to match strict FP nodes for
floating-point extend operations with a register as source, this
did not work for operations with memory as source.

That is because from regular operations, this is represented as
a combined "extload" node (which is a variant of a load SD node);
but there is no equivalent using a strict FP operation.

However, it turns out that even in the absence of an extload
node, we can still just match the operations explicitly, e.g.
   (strict_fpextend (f32 (load node:$ptr))

This patch implements that method to match the LDEB/LXEB/LXDB
SystemZ instructions even when the extend uses a strict-FP node.

llvm-svn: 364450