When folding loads into ops, immediately replace uses of the op with the

When folding loads into ops, immediately replace uses of the op with the
load.  This reduces number of worklist iterations and avoid missing optimizations
depending on folding of things into sext_inreg nodes (which aren't supported by
all targets).
Tested by Regression/CodeGen/X86/extend.ll:test2

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@24712 91177308-0d34-0410-b5e6-96231b3b80d8